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Integration of Sneak Analysis with Design

1. In this manner non relevant clues are filtered and wherever possible the user is directed to specific areas of the circuit under concern As an example consider the power to power tie highlighted in the schematic of Figure 3 4 The circuit is a portion of the FB111 A B Pivot Pylon Weapon Station Circuitry that underwent a conventional SCA circa 1975 BOEI75 The power tie was identified by SCAT as shown by the screen in Figure 3 5 As shown in Figure 3 6 the user is also offered assistance by way of an explanation message and a possible solution These aids are directed toward less experienced users 24 AAA RARE oo e Ren ir e O O t i o iie i c e E lp o c Ja Bei lo BLOCKMOUSE MISSILE i PPLICATION DISPLAY reds net src to snk reverse current path 1 of 9 path ref number 1 K4 S P30 P19 P1 P2 P18 LP6 K1 C P15 P6 P5 PZ1 P31 12 NOTE Asterisk indicates a reverse current device Deleted Parallel Parts 1 Deleted Switch Related Parts 1 Deleted Miscellaneous Parts Use up down arrows to scroll answers NSUER Display Next Path Display Previous Path preuious finalyze Current Path analyze Delete Current Path delete Print Path print Return to Menu return Space to Mark ALT A Abort F2 Scroll Display F10 Command Menu Figure 3 3 SCAT Reverse Current Path Display 25 eAMIIOSNIO 2593 3 34 SUNY TIT HI7TS B A0 SISA DINO 0591281231 156 Coig Og Ain AMLININID
2. Sink snk Deleted Parallel Parts J Deleted Suitch Related Parts J Deleted Miscellaneous Parts _ AMA NSWER Please choose desired option Execute Sneak Path Search delete Delete Parts unde lete Undelete Parts change_source Change Source change_sink Change Sink return Space to Mark ALT A Abort F2 Scroll Display F10 Command Menu Figure A 10 Sneak Input Data Menu 67 M1 Conmands PPLICATION DISPLAY demo net src to snk reverse current path 1 of 2 path ref number 4 EK1 S SZ K1 C NOTE Asterisk indicates a reverse current device Deleted Parallel Parts 1 Deleted Switch Related Parts J Deleted Miscellaneous Parts 1 ALT A Abort Y PI gt no er pu en m0 108 REIP ecat Use up down arrows to scroll answers NSWER Display Next Path Display Previous Path previous Analyze Current Path analyze Delete Current Path delete Print Path print Return to Menu return Space to Mark F2 Scroll Display F16 Command Menu Figure A 11 Reverse Current Path 1 Figure A 12 Path 1 Highlighted on Schematic 68 Mi Commands PPLICATION DISPLAY demo net src to snk reverse current path 2 of 2 path ref nunber 6 I01 S2 LP21 NOTE Asterisk indicates a reverse current device Deleted Parallel Parts j Deleted Switch Related Parts Deleted Miscellaneous Parts J Use up down arrows to scroll answers NSUER Di
3. directional i e capable of conducting curre in either direction depending upon the circuit s switching state Each reverse current path identified is displayed to the user as an ordered list of device reference designators keyed to the circuit schematic The user traces each reverse current path on the schematic to determine the path s validity i e whether system operation precludes the assumed switching state required by the path and its significance i e its effect on mission success personnel safety equipment damage etc Reverse current paths are identified from the net list by the C program SCA The knowledge base SCAIN enables the user to specify source and sink nodes and to set the switching state of any device The knowledge base SCAOUT displays the reverse current paths and provides assistance in evaluating their validity SCAOUT also allows the user to mark any reverse current path as invalid and regenerates the paths to eliminate the marked one and all others solely dependent upon it A complete analysis requires that the sneak path search be re run for all combinations of source and sink nodes involving power supplies sources and grounds sinks Design Concern Analysis differs from Sneak Path Search in that the former is a highly interactive consultation and can be applied to any analog digital or combined analog digital circuitry The analysis identifies problems associated with 1 circuit configurations involving specif
4. functional clues integration with FMEA and FSCA examples y 8 MIL STD 1543B Reliability Program Requirements for Space and Missile Systems 25 October 1988 i Gen Para 204 2 8 2 specifies application of design clues Appendix C of std during hardware FMEA Para 204 2 8 6 specifies application of functional clues Appendix B of std during functional FMEA Task 205 specifies i performance of a Design Concern Analysis DCA using a design concern d j clue list Appendix D of std 9 MIL STD 785B Reliability Program for Systems and Equipment Development and Production 15 September 1980 Task 205 defines SCA as a method to identify latent paths which cause unwanted functions to occur or which inhibit desired functions All i components shall be assumed to be functioning properly Paragraph 50 2 3 2 describes SCA as applicable to critical systems not to off the shelf l hardware Also describes SCA as being costly and performed too late in the development cycle Paragraph 60 1 of this standard references data item DI R 7083 SCA Report 10 J Miller Sneak Circuit Analysis for the Common Man RADC TR 89 223 October 1989 Presents a manual procedure for SCA in a simple easy to follow format Includes a description of common design errors which can lead to sneak circuits as well as guidelines for identifying functional and device oriented sneak problems The procedure is ta
5. top down deductive procedure for identifying causes of system failures The analysis utilizes diagrams referred to as fault trees for depicting the dependency of higher level failure events on lower level events Common cause failure analysis is a similar top down procedure that identifies single failure events caused by the occurrence of multiple events The analyses where they are being conducted can be used to identify the functions and components to be subjected to SCA By narrowing the scope of the latter substantial cost savings are possible Failure modes and effects analysis FMEA shares important techniques with sneak circuit analysis including the use of network trees FMEA is a bottom up procedure for inferring the higher level effects of postulated lower level failures The effects may be used as a baseline for an extended SCA where the analysis is required to identify sneak circuits in the presence of one or more faults Here a sneak circuit can compound the effects and increase the criticality level Also some of the effects of failures may be duplicated by improper or unintended operation of non failed equipment Thus sharing of data may be beneficial Where fault tree analysis is not being conducted the components associated with catastrophic and major failures in FMEA can be used as a candidate list for SCA It is important to realize that FMEA is frequently a basis for the design of built in test and operational test equipm
6. RS 16 3 AUTOMATED SCA oscar ORE RO e a Rs 20 3 1 Oy rvie EEN 20 3 2 Description ai a EE el A e Sat ree Oe 21 3 2 1 Schematic Capture Net List Generation o o 23 22 2 Sneak Path Analysis lesa E er OO OLOR Ma Re Ree eg 23 3 2 3 Functionally Oriented Design Concern Analysis 24 3 2 4 Device Oriented Design Concern Analysis LL 28 3 3 Test Result az ERA UOI O 28 d CONTROL AND MONITORING OF SCA oooooooooooooooo ooo 34 41 Background EE EE 34 42 Recommended Procedures cinc a e es 35 42 1 SCA Follow up occa NEE ct Ss we AC ON a ee ee RA 8 35 4 2 2 SCA Effectiveness cessa AA AA aR ES 35 42 3 SCA Cost Effectiveness ains Re a a NR a 36 42 4 SCA Thoroughness xeu e ww td e a ad 36 43 Proposed Data MOMS es ia a O A a 37 4 3 1 Modifications to DI R 7083 Ref Appendix B 37 4 3 2 Modifications to DI R 7080 Ref Appendix C lt 37 44 Automating SCA Evaluations cce 38 4 4 1 Database File 1 Vehicle System Application File 38 4 4 2 Database File 2 Analysis Test File o o eee eee 39 4 4 3 Summary of SCA Data Collection and Analysis Requirements 41 5 RECOMMENDATIONS FOR FURTHER STUDY oooooooooo o 43 5 1 CAD TIMES la de a da dos a 43 5 2 Expansion and Integration of the Knowledge Base 43 Do Net List Format ac cs ick eed oC CERA X OR US RR RT CEN 44 ebe e A F
7. SCAT SCA Tool SOW Statement of Work EXECUTIVE SUMMARY This report documents the results of a two year study to make sneak circuit analysis SCA more effective by simplifying the procedure and integrating it into the design process This effort entailed 1 the conduct of a literature search and an SCA user survey to ascertain current methodologies and techniques associated with SCA and its support of other reliability analyses 2 development of a simplified manual procedure which provides design rules for avoiding sneak paths and guidelines for identifying common types of sneak conditions and 3 development of an automated version of the procedure integrated with a popular computer aided design CAD tool The literature search identified two comprehensive non proprietary sources for SCA clue lists In all six clue lists were evaluated with clues falling into one of two broad categories Those applicable to specific topological patterns in switching circuits and used for identifying sneak paths Those applicable to specific circuit devices or functions and used for identifying design concerns and to a lesser extent sneak paths The most significant problems associated with the clue lists are their lack of structure to facilitate culling inapplicable clues inclusion of subjective areas in what should be an objective analysis and inclusion of topics better handled by other analyses In the user survey 42 potential SCA use
8. STD 1629 severity classifications Possible dispositions include problem corrected falsely identified ignored due to time budget constaints Include as applicable CCB date CCB number CCB Action Estimate the cost in terms of dollars and labor 42 Chapter 5 RECOMMENDATIONS FOR FURTHER STUDY mE This study has resulted in the development of a manual and an automated procedure for sneak circuit analysis The procedures have been streamlined for simplicity and integrated into the design activity The scope of this effort however did not include optimizing this integration and several areas of research remain to be addressed in this connection 5 1 CAD Integration A more effective user interface can be attained by incorporating SCA results within the computer representation of the schematic For example sneak paths and devices or areas of the circuit that are the subject of design concerns could be highlighted Additionally availability of the schematic editor during SCA would permit real time evaluation of solutions to sneak problems However inherent limitations of the MS DOS operating system and of the selected schematic capture tool preclude this type of integration Alternative implementations should be investigated As workstations and personal computer based multi tasking operating systems become more widely accepted the above difficulties can be overcome by hosting SCAT in those environments An additional
9. UOFETATG IFRAVOD ON SOK UUOg UPTITIM UOTSTATG IFPAUVOD PA BRO E m esed SEET eer ves woisTAT 3YeAuon ON Sax 891 uojdag pieursdg Garg ues VOFSTFATG IIPAUOD Sat Sat Sat xL u ienoy uorjeiodo ooedsoiov b rsou UorT3 3odio2 Uorya iodios uol 3e1odi05 UOFIPIOdIO ooedsoidov 30936005 qog SrUUSQ ITEM peuken A11e7 Wo ueq zequen DCK UPIICUUPN ueby rn ueqqequen BUT Joy UIMO 33eqUeW Uorieiodio2 e2 deoiov 89A Z2I19H5 6o 21eu5 oda UO3DUTUuS M 2Jo31enDpeoH VSVN rass arene wos tenprarper usriw201 Xu duog 1 EISE u v A u o Ba AN H 9 Q o 4 o z 9 gt 11 Table 2 3 SCA Survey Summary Number of Area Respondents Comments Procedure Circuit 4 Three perform it manually One has a fully automated system Partitioning Network Tree 3 Two have manual and computer aided procedures One has a Generation fully automated procedure Automation 2 One indicated the need for a minicomputer or mainframe Both Environment indicated that SCA work stations are under development Data Entry Schematics 4 All use manual and computer aided procedures Net Lists 3 All use manual and computer aided procedures Functional Nets 3 All have manual procedures One has a computer aided procedure Timing Data 3 All have manual procedures Two have a computer aided procedure Clue Types Sneak Path 4 All
10. analysis Provide summary of total sneaks identified number corrected number falsely identified etc Estimate total number of components in each system or subsystem analyzed Identify when SCA was accomplished prior to CDR after CDR but prior to FCA after FCA 10 4 Status Provide a table which lists all sneaks identified through SCA This shall include information fror reports such as Sneak Circuit Report Design Concern Report Drawing Error Report Sneak Software Report Software Design Concern Report Software Document Error Report etc As a minimum the list shall include the following data for eacn sneak Date Provide date the sneak was identified Sneak Nurber Provide the contractor assigned reference number for the identified sneak Title Provide title of the identified sneak System Subsystem Applicable system or subsystem in which sneak is located Sneak Category Identify the sneak category as 3 Sneak Path Current energy or logical sequence 1s caused to flow along an unexpected path or in an unintended direction Sneak liming Events occur in an unexpected or conflicting sequence Sneak Inuication These cause an ambiguous or false display of syster Exhibit B 2 Proposed Revision to DI R 7083 Page 1 of 3 80 BLOCR 7 CONTINUED Phase Full Scale Development Phase ard or Production Phase through DD form 250 sign off 7 5 The CDRL should specify initial submittal prior to Critical Design R
11. and Electrical Generating System AFATL TR 75 133 AD B031159L Sept 1975 Primarily a digital SCA at both the system and device IC level Most of the identified sneaks resulted from postulated failures as in an FMEA J L Vogas Verification of Hardware Software Integration Through Sneak Analysis paper presented at American Defense Preparedness Association Forum 81 27 October 1981 Includes SCA and Software Sneak Analysis background information and simplified examples of combined hardware software sneak analysis J L Wilson and R C Clardy Sneak Circuit Analysis Application to Control System Design ntegrity in Electronic Flight Control Systems AGARD AG 224 AD A041042 April 1977 pp 12 1 6 Includes a description of the Mercury Redstone launch failure in 1961 that was attributed to a sneak circuit 91 4 PORRA AA Glossary The definitions provided for the following terms apply only insofar as the terms are used in this report Break Before Make Refers to any switching device e g switch relay contactor having a multiple contact arrangement such that upon being switched the selected contacts will close i e make only after the de selected contacts open i e break Clue A statement or question directed toward the SCA analyst regarding the presence of a specific condition that past experience has shown to have caused a sneak circuit Clues are of two basic types 1 Those assoc
12. compiling sneak clues from several sources see section 2 2 1 selecting clues most relevant to identifying sneak problems and generating a concise yet thorough guide for their application The guidelines resulting from this effort are categorized as follows 1 Design rules for an engineer to follow early during development to avoid designing circuits prone to sneak conditions 2 Functional guidelines for an engineer or reliability analyst to identify sneak c nditions appearing in power distribution circuits power and signal timing and status indicator circuits 3 Device guidelines for an engineer or reliability analyst to identify sneak conditions caused by circuit devices including relays transistors and linear and digital integrated circuits Each design rule and guideline contains a brief statement of the problem being identified a recommended solution a supplementary paragraph describing the problem in greater detail and an illustration of the problem and solution The guidelines also include a descriptor for categorizing the targeted circuitry The design rules are the most cost effective of the three items for addressing sneak problems during design and for this reason are emphasized by the procedure It is far easier and less costly to avoid sneak circuits through proper design techniques than to identify and correct sneak circuits after the design has been completed While the design rules are intended for an engineer res
13. design concerns The user must respond to each query for the analysis to continue Queries addressing specific devices reference those devices by their schematic labels Where switch or relay contacts are referenced the specific terminals being addressed are indicated along with the device label eg Kl commonl The terminal identifiers are the same as those appearing in the OrCAD device library As each design concern is identified an appropriate message is displayed The user may request an explanation of the design concern a possible solution or re display of the original message The user is given an option for printing out identified design concerns at the conclusion of each guideline session A 7 SCAT APPLICATION EXAMPLE Note In the following procedure data to be entered appear enclosed in chevrons e lt data gt Entries must be typed lower case followed by a carriage return Do not type the chevrons Entries to be selected are enclosed in quotes i e selection Entries are selected by using arrow keys to highlight the selection and pressing ENTER The selection unknown is not operative Figures for this section appear at the end of this section The example references the schematic DEMO Figure A 6 SCAT will be used to ideniify a sneak path shown highlighted in Figures A 12 and A 14 and a power to power tie shown highlighted in Figure A 28 Entry of specific switch contact timing configurations e Make Before Break o
14. facility 2 The terminals of all in circuit voltage sources e g batteries must be labeled using the OrCAD SDT module port facility 3 Any device appearing in the OrCAD SDT DEVICE library may be used The devices are shown in Figures A 3 and A 4 In addition any IC may be used as long as it is referenced by the prefix U see item 4 4 Schematic reference designations for circuit components must use the default label prefix provided by OrCAD SDT e g R for resistor one K for relay U for IC etc Refer to Figures A 3 and A 4 for the default reference designator of each device 5 Any labeled power ground or signal path may be specified as a source or sink for Sneak Path Analysis This may be done 1 while running SCAT by specifying the path name or 2 while editing the schematic by labeling the desired module port as SRC or SNK The OrCAD SDT schematic error checking utility ERC can be used to check circuit connectivity for shorts between outputs inputs with no driving source unconnected pins and other common wiring errors The net list is generated using the OrCAD SDT NETLIST utility The special format FlatEDIF must be specified when invoking the utility 53 SISI 39N LVOS 03 SIOQUI S DNBUISYIS CV 9ANBLA 219n00u4 AVNIMVD DILVHIMIS OMIMONIOS PHL O1 SONMOASISVVO OM ng T 4 95 NO 134308 Wc 1 310N M 5 WNO 13450 o d 134509 e kW Ndas wW34y0 E lt m M 134 0 j i d
15. greater insight into past programs so that conclusions may be drawn These might take the form of recommending analyses or wests which are best suited to a specific application Others might recommend which analysis to perform in the eariy phase of a specific program Still another might be a recommendation of tests which identify the greatest number of problems during later program phases In conclusion this study will have succeeded if it has stimulated ideas on how collecting organizing analyzing and utilizing past data can improve the system acquisition process 41 Table 4 1 SCA Report Checklist Major Concerns Summary Information Has the described analysis methodology been Has the analyzed circuitry been defined Has the time phasing for the analysis been identified Have the results of the analysis been summarized Data Required for each Sneak Has the severity of each sneak been identified Has the disposition of each sneak been identified Have tracking data been provided Has the cost of correcting the problem been estimated Comments Procedures should be described and computer resources hardware and software identified Identify the system subsystem s and estimate their component size State when the analysis was performed in terms of program milestones Oe prior to CDR Include total number of sneaks identified total number corrected total number rejected Use MIL
16. have manual procedures addressing analog and digital circuits One has a computer aided procedure Sneak Timing 4 All have manual procedures addressing analog and digital circuits Two have a computer aided procedure Design Concerns 4 All have manual procedures addressing analog and digital circuits Two have a computer aided procedure Design Integration FMECA 5 Tolerance 4 Fault Tree 5 Respondents indicated the use of SCA results or tools Power amp Load 2 i e trees to aid the performance of these analyses Grounding 2 Safety 3 Application Phase 5 One respondent has applied SCA prior to Full Scale Engineering Development FSED All have applied SCA during FSED 12 The summary table indicates that The current prevalent procedure for SCA consists of automated formatting and partitioning of schematic and net list data semi automatic generation of network trees and manual application of sneak clues and design concerns Efforts are underway for reducing the requisite computer resources from a mainframe to a workstation Functional networks block diagrams are rarely analyzed The most prevalent types of analyses for which SCA databases and results are shared are FMEA and fault tree analysis Conventional SCA techniques are based upon the generation and utilization of network trees Trees aid the analysis by segmenting the circuitry into small topologically related units omitting extraneous detail and dr
17. in the bibliography of this report The performance of SCA is centered on the use of clue lists serving as checks or guides for the analysis Two broad categories of clues can be distinguished Clues applicable to specific topological patterns in switching circuits and used for identifying sneak conditions Clues applicable to specific circuit devices or functions and used for identifying design concerns and to a lesser extent sneak conditions Two of the references identified by the literature search are comprehensive non proprietary sources for SCA clues This is significant since sneak clue lists have traditionally been considered proprietary and were not published Reference NP3634 includes 106 clues in three major categories 1 tree topograph 2 piece part and circuit configuration and 3 design concerns Clues in the latter category are accompanied by explanations to assist less experiericed analysts Reference MS1543B includes 128 clues also in three major categories 1 functional 2 design oriented and 3 design concern The design oriented clues are written so as to enable identification of sneak conditions i e paths timing indications and labels without reference to network tree topographs An evaluation of these clue lists and four others obtained during the course of this study covering approximately 150 unique clues revealed that while the clue lists provide a valuable guide for relatively inexperienced p
18. path begins at the point labeled SRC passes through relay K1 switch contact switch S2 and back through relay K1 coil before terminating at ground The sneak results in an oscillatory condition that alternately energizes and de energizes the relay SCAT designates the path by listing the affected devices in their order of appearance along the path i e K1 S S2 K1 C The SCAT program requires only a few minutes to run the actual time being dependent upon the size of the circuit analyzed As in the manual procedure the SCAT user must understand the operation of the circuit under analysis in order to evaluate identified sneak paths and to respond to prompts addressing design concerns The program consisting of an expert system knowledge base augmented by external code runs on IBM PC XT AT or 80386 class microcomputer running under MS DOS The user must provide the M 1 Teknowledge Inc expert system inference engine software environment to run the SCAT program Input to SCAT consists of net lists generated by a popular commercially available schematic capture program OrCAD SDT III that must also be provided by the user This report concludes with an evaluation of current SCA control and monitoring procedures and in this regard recommends revision of two existing DIDs The revisions address data requirements tur performing follow up SCA effectiveness and thoroughness studies Examples of the revised DIDs are provided in the appendices
19. representative for specific program discipline Program Management Reliability System Safety Engineering etc 34 SCA The corresponding DID DI R 7080 Reliability Status Report is used to monitor and evaluate a contractor s progress and accomplishments in conducting the reliability program for the applicable contract end item s SCA is generally conducted by the reliability or safety organization within a project team and is performed in accordance with Task 205 of MIL STD 785B Sneak analysis activity is reported via DID DI R 7083 Sneak Circuit Analysis Report The DID is invoked for periodic reporting on the status of all current and previously submitted problem reports i e Sneak Circuit Report Design Concern Report Documentation Error Report all of which are in the contractor s own format and for a final report summarizing the results at the conclusion of the SCA Conventional SCA is based on detailed system drawings and computer programs These data may be available late in program Demonstration Validation Phase but would more likely be available late in the Full Scale Development Phase Consequently resultant data from DI R 7083 may or may not be available at significant program decision making milestones Sneak report monitoring by the PA depends to a great degree on each contractor s reporting and tracking process and to a greater degree on the contract statement of work requirements In general identifi
20. system shell all running under MS DOS and controlled by a DOS batch file program The user must have a copy of MI to run SCAT Hardware requirements are an 80286 or 386 based personal computer with a minimum of 640K RAM and a 10MB hard disk an IBM XT class machine can be used but performance in terms of circuit size and analysis speed will degrade E EXAMPLE A EXAMPLE B EXAMPLE C NOTE PARTITION BOUNDARIES ARE INDICATED BY THE BROKEN LINES Figure A 1 Proper Circuit Partitioning 47 Input data for SCAT is a schematic net list Ge a list of device interconnections formatted in EDIF version 1 1 0 as generated by the OrCAD SDT III schematic capture tool OrCAD a popular commercially available MS DOS based CAD package is not bundled with SCAT The SCAT installation procedure is as follows 1 Insure your computer s system configuration file permits 8 or more files to be open concurrently i e FILES x where x gt 8 For MS DOS the default value of the FILES parameter is 8 2 You must have a copy of M 1 version 2 1 or a later compatible release installed on your hard disk in accordance with the vendor s Teknowledge installation instructions 3 Copy all files from the SCAT program floppy disk on to your hard disk in the same directory as M 1 4 Copy all net lists to be analyzed into the same directory as M 1 Net lists must be generated by OrCAD SDT III using the FlatEDIF data format A 4
21. trees sneak circuit reports Boeing Aerospace Sneak Circuit Analysis of F 104 DCU 201 GWM 4 Nuclear Weapon Controller Tester Subsystem AD B103044 February 1978 Sneak circuit design concern and drawing error reports The reports include USAF follow up action Note Distribution of this document is limited to DoD or U S DoD contractors D L Buratti and S G Godoy Sneak Analysis Application Guidelines RADC TR 82 179 AD A118479L June 1982 General SCA background with detailed information on project management Includes a comparison of SCA with related analyses and an overview of interfacing SCA with CAD A H Carter K T Budnick and S R Douglass Computer Produced Drawings for Circuit Analysis Proc RAMS January 1985 pp 224 229 Overview of an automated system ECAD SCA for drawing network trees Also performs automated partitioning and device modeling R C Clardy Sneak Circuit Analysis in J E Arsenault and J A Roberts ed Reliability and Maintainability of Electronic Systems Computer Science Press 1980 pp 223 241 89 Historical overview of Sneak Analysis development and simplified examples of its application Includes SCA Digital Logic SCA and Sneak Software Analysis along with suggested uses of network trees by other analyses T Jackson Integration of Sneak Circuit Analysis with FMEA Proc RAMS January 1986 pp 408 414 i A description of Functional SCA FSCA
22. Approved for public release distribution unlimited 13 ABSTRACT Maxarum 200 worda This report documents the work in the creation of a software package to be used by a design engineer to prevent sneak circuit failures in a new design Sneak Circuit Analysis for the Common Man was an interim report that was issued representing the manual procedure for identifying possible sneak circuits This repert presents the automated version to be used on an IBM PC under MS DOS The Sneak Circuit Analysis SCA software package uses the ORCAD JII schematic capture program to analyze the circuitry SCA will search for potential sneak paths and identify them for the user SCA will then offer suggestions to the user to correct the design weaknesses The software package handles analog as well as digital circuits and for very large net works a sectional analysis is possible ke vc Ee Sneak Circuit Analysis sneak paths timing indications labels 108 16 PRICE CODE clue list 17 SECURITY CLASSIFICATION 18 SECURITY CLASSIFICATION 19 SECURITY CLASSIFICATION 20 LIMITATION OF ABSTRACT OF REPORT OF THIS PAGE OF ABSTRACT UNCLASSIFIED UNCLASSIFIED UNCLASSTFTED UL i P Stanoara rorm 290 880922 NSN 7540 01 280 5500 Prescrped by ANSI Sui 229 18 299 01 amm ae awe HOA Mor PREFACE This is the final rezort for a two year study entitled Integration of Sneak Analysis with Design conducted by SoHaR Incorporated prime contractor an
23. IMINARY DESIGN REVIEW 10 PROGRAM MILESTONE DATE FOR CRITICAL DESIGN REVIEW 11 PROGRAM MILESTONE DATE FOR FUNCTIONAL CONFIGURATION AUDIT 12 PROGRAM MILESTONE DATE FOR PHYSICAL CONFIGURATION AUDIT PROGRAM MILESTONE D ATE FOR FORMAL QUALIFICATION REVIEW Figure 4 1 Vehicle or Application System File Field 1 Vehicle or Application This field contains information about the top level acquisition program This could be an aircraft manned or unmanned fixed or rotary wing a satellite a launch vehicle ground systems a new avionics or communications suite a computer or even a circuit board This field should contain the highest level title of the acquisition program or project Field 2 Designator This field contains the program designator for the vehicle or application in field 1 Using the examples above this could be B 2 BQM 126A or AH 64A aircraft DSCS 3 spacecraft Titan 4 launch vehicle OTH B ground systems AN ALQ 172 avionics Computer and circuit board could be listed by part number Field 3 Program Cost This field contains the funded cost of the program This should reflect costs up to but not including production costs Eield 4 Contract Cost for Each Analysis and Test This field which may require multiple subfields contains the contract line item cost for and title of each analysis and test If cost data are not available the field could be left blank When sufficient data on similar analyses or tests are coni
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25. NALINIS SANA IN MOM d ZOALS BCOITHIGS WOUd QJ12U4IXJ LINSUID rt S310M CEZITD gt ZII 11 H232u8 DIU tos o ae EE ES Tos o AN Old JOA uonejg UOdEIM ULIL WWMOg p g INZ CYYZAE COTZIRO CEZIAs gt CES CAZA 3710125 XI Ta 26 Hi Commands WARNING Check for possible pouer to pouer tie between ess_2Budc and x_28udc_mau_firel when switching devices are closed The sneak path s comprises the following devices A path 1 CB2 P49 P50 K4 P23 P18 NSUER Choose one of the follouing options explanation solution varning nessage return unknoun Space to Mark ALT A Abort F2 Scroll Display F10 Command Menu Figure 3 5 SCAT Power Tie Display y Mi Commands PPLICATION DISPLAY fi pouer to pouer tie betueen supplies can result in damage to the supplies or to the load This problem can arise if primary and backup supplies are connected to a common load through a make before break switch pg1 For DC power add diodes to isolate the supplies Schottky diodes can be used to ninimize the diode voltage drop For AC Cor as an alternative for DC use a double throu relay or suitch having a break before make contact arrangement to select either supply and provide adequate capacitance at the load to maintain the supply voltage during suitch over QUEST ION NSWER Choose one of the following options explanation solution varning_nessage return unkno
26. OLSTHM INL ma ZA tiw worelve LIO ame m VE te Awsiive ssent aht o o0 sie tat A 6 SCAT OPERATING PROCEDURE Note In the following procedure data to be entered i e entry followed by carriage return appear enclosed in chevrons i e lt data gt Entries must be typed lower case Do not type the chevrons Entries to be selected are enclosed in quotes i e selection Entries are selected by using arrow keys to highlight the selection and pressing ENTER The selection unknown is not operative l Enter lt scat gt A net list file name will be requested 2 Enter the name of the net list file for the circuit to be analyzed Use lower cose text The file will be retrieved and pre processed This will typically take 5 10 seconds If the file is not found a bad file message will appear and a net list file name re requested Otherwise the main menu will appear and the user is requested to select the type of analysis to be performed 3 Before proceeding to select Sneak Path Analysis sneak or Design Concerns Analysis design the user has the option of modifying switch relay and capacitor models to agree with their engineering specifications or usage The contact arrangement for multiple throw switches and relays see Figure A 5 can be specified as Break before Make or Make before Break In the former the default case when the switch or relay is toggled the newly selected path is established after the old
27. OUND SCA has been in use for over 20 years the first major computer aided version having been developed for the NASA Apollo program in 1967 by the Boeing Company CLAR76 The original application of SCA was for switching and relay networks for engagement and disengagement of control functions such as those used in automatic pilots and in missile and spacecraft systems These applications are referred to as electro mechanical circuits in MIL STD 785B where SCA is specified as Task 205 in this report the shorter terms switching circuits or relay circuits are used the two expressions are considered synonymous The change in terminology also recognizes that relays are no longer exclusively electro mechanical devices 2 1 Conventional Techniques The techniques for identifying topological sneak paths in switching or relay circuits are applicable to all functions that evaluate Boolean variables exclusively Such circuits may be comprised of manual or sensor operated switches electro mechanical or solid state relays or combinatorial digital logic circuits but not sequential or memory dependent ones The logic circuits are modeled by their switching circuit switch and diode equivalents Functional paths such as those between relay coil and contact and between poles of a multiple pole switch are also modeled The primary objectives of SCA in switching circuit applications are to uncover sneak problems in four principal areas Sneak Paths Un
28. SCAT OPERATION This section provides a detailed description of SCAT operation This material is not required for understanding thu operating procedure that appears in section A 6 A complete list of names of SCAT programs and knowledge bases appears in Table A 1 Documented listings of the source code and knowledge bases appears in volume II of the final report A list of temporary data files generated by the programs appears in Table A 2 SCAT programs controlled by the DOS batch file SCAT BAT are shown by the diagram appearing in Figure A 2 The program names appearing in the figure are referred to in the following discussion Upon invoking SCAT the net list entry screen generated by the knowledge base SCAFILE enables the user to specify a net list file to be analyzed At this point control returns to SCAT BAT which invokes the C program EDIF2M1 This program reads the specified net list file and outputs a reformatted version in two files DEVS SCA and JOINS SCA for use by the M 1 knowledge bases If the net list file is not found SCAT BAT calls the M 1 file SCAFILEB which displays an appropriate error message and re reouests a net list file name Otherwise SCAT BAT calls M 1 file SCAMENU which generates the main menu for SCAT 2 Electronic Data Interchange Format as specified by EIA Interim Standard No 44 Version 2 0 0 was not available at the time this effort was undertaken 48 Table A 1 SCAT Programs Knowledge Bases an
29. SPLAY Select SNEAK PATH SEARCH or DESIGN CONCERN ANALYSIS Pr or to selection modify suitch relay and capacitor models if necessary All switches and relays vill be modeled as BREAK BEFORE MAKE unless otherwise specified All capacitors will be modeled as OPEN circuits unless otheruise specified Type of switching for relays and switches MAKE BEFORE BREAK M BREAK BEFORE MAKE B Append MANE BEFORE BREAK to list delete M Delete MAKE BEFORE BREAK to list return Return to Main Menu unknown Space to Mark ALT A Abort F2 Scroll Display F10 Command Menu Figure A 20 Switch Relay Model Menu 72 Mi Commands PPLICATION DISPLAY Select SNEAK PATH SEARCH or DESIGN CONCERN ANALYSIS Prior to selection modify suitch relay and capacitor models if necessary All switches and relays vill be modeled as BREAK BEFORE MAKE unless otherwise specified All capacitors will be modeled as OPEN circuits unless othervise specified NSWER Enter name of MAKE BEFORE BREAK switches or relays to be added Type done when finished Enter lowercase expression AETA Abort FZ Scroll Display F10 Command Menu Figure A 21 Switch Configuration Menu HI Commands PPLICATION DISPLAY Select SNEAK PATH SEARCH or DESIGN CONCERN ANALYSIS Prior to selection modify suitch relay and capacitor models if necessary All switches and relays uill be considered as BREAK BEFORE NAKE except fo
30. aT wwe ee ee mm em emm M ER ERED rete ee eee re E ERKKEEERKKEKKKEKEANT pile LUPA RADC TH 30 109 Vol I of two Final Technical Report June 1990 SoHaR Incorporated 4 uU H E D E Jeff Mille DATAS d 4 D Y tuc EL a WE D Ka PT mee Lage A dt q ee d LEE BRS Wo 6 ros 3j E X separ ood P pu pe BE Yu ing 5 td DT LER a Sue e ay Dm APPROVED FOR PUBLIC RELEASE DISTRIBUTION UNLIMITED 90 09 26 086 Rome Air Development Center Alr Force Systems Command Griffiss Air Force Base NY 13441 5700 This report has been reviewed by the RANC Public Affairs Divicion PA and is releasable to the National Technical Information Services NTIS At NTIS it will be releasable to the general public including foreign nations RADC TR 90 109 Vol I of two has been reviewed and is approved MH publication APPROVED 2 ege b p BRUCE W DUDLEY Project Engineer APPROVED QUUM Lag JOHN J BART Technical Director Directorate of Reliability amp Compatibility eS oh FOR THE COMMANDER 4 JAMES W HYDE III Directorate of Plans amp Programs t 3 If your address has changed or if you wish to be removed from the RADC mailing list or if the addressee is no longer employed by your organization please notify RADC RBER Griffiss AFB NY 13441 5700 This will assist us in maintaining a current mailing list Do not return copi
31. actor s Reliability Program Plan b Summaries of the status of previously reported programs which were unresolved at the close of the last reporting period C A summary table of all identified design problems The list shall be on two parts 3 Part 1 will list current open problems and shall contain a 5 tc d e 0 Serial number assigned to identify each problem Date on which proble was first detected Short statement identifying the problem and its effect Activity assigned to solve the problem Expected resolutior and date to be achieved Short statement of accomplishment to date or a cross reference to other reports 2 Part 2 will begin on a new page and contain a surmary table of al problems identified during the progra The list shall contain a b te e Seria number assigned to identify each problem Date on which proble was first detected Date the proble was resolved Title of the probie report Syster or subsyste in which the problem was located Exhibit C 2 Proposed Revision of DI R 7080 Page 1 of 2 86 oM e ee eR P o ee A M H M ve RESTA lt VAS Le HUNE BLOCK 10 CONTIALED f Brief description of each problen sneak circuit unintentional state transition component failure etc g The analytical tool or test method used to identify each problen Snea
32. ained within the database RADC could determine the percent of program costs expended on analyses and test and then estimate values for each blank subfield to complete each record Fields 5 13 Program Milestone Dates These fields initially contain scheduled dates and should be updated with actual milestone dates 4 4 2 Database File 2 Analysis Test File This file contains data related to problems identified during analysis and test The two common fields linking it with File 1 are the Vehicle or Application field and the program 39 Designator field Remaining fields in File 2 contain analysis or test data as well as data to identify key functional systems Data used to develop this file are contained in DI R 7080 as modified by this study Figure 4 2 below identifies pertinent fields in each Analysis Test record Detailed information about record fields in File 2 follow FILE 2 ANALYSIS TEST SUMMARY FILE FIELD DATA VEHICLE OR APPLICATION DESIGNATOR TYPE OF ANALYSIS OR TEST DATE PROBLEM IDENTIFIED TITLE OF PROBLEM REPORT VEHICLE OR APPLICATION SYSTEM VEHICLE OR APPLICATION SUBSYSTEM HARDWARE OR SOFTW ARE HAZARD CATEGORY DISPOSITION MANHOURS TO CORRECT PROBLEM COST TO CORRECT PROBLEM 1 2 3 4 D 6 7 8 9 Figure 4 2 Analysis and Test Data File Field_1 Vehicle or Application This field contains information about the top level acquisition program This could be an aircraft manned or u
33. ar The page number at the end of the explanation references the page in the report SCA for the Common Man where additional information regarding this concern can be found 61 16 17 18 19 20 21 22 23 24 25 At Select solution A solution for the concern will appear Note that as the text appears earlier messages scroll off the APPLICATION DISPLAY window These earlier messages can be retrieved by pressing function key F2 and using the arrow keys to scroll back through the text To continue the analysis F2 scrolling must be disabled by pressing the ESCAPE key Select warning message The original warning message is repeated Select continue A warning regarding switching devices in ground paths appears Select return where available as a choice or press ALT A Alt key and the letter A at any time to interrupt the analysis and return to the design concern type menu Alternatively the remaining functional design concerns may be viewed by repeatedly selecting continue until no further concerns have been identified At that point a hardcopy of the identified design concerns may be requested or return may be selected to return to the design concern type menu At the design concern type menu Figure A 19 select device guideln The device oriented design concern analysis will commence A query regarding loads of specific transistors will appear Select yes A query regar
34. are available from the reverse current path menu 45 1 Display the next path Paths are consecutively numbered for reference 4 5 2 Re display the previous path The path queue is circular 4 5 3 Mark a reverse current path for deletion This is required if user determines the path to be operationally impossible e g due to forbidden switching states 4 5 4 Unmark a marked path This option is available only when a path has been raarked 4 5 5 Display deleted paths This option is available only when one or more paths have been deleted The deleted path menu is similar to the reverse current path menu 58 4 5 6 Regenerate paths This option is available only when a path is marked for deletion or a deleted path is marked to be undeleted 4 5 7 Print the path currently displayed for hardcopy reference 4 5 8 Request computer aided analysis of the sneak path The system will prompt the user for basic circuit related information necessary to evaluate the significance of the sneak path in terms of inhibiting desired functions or causing undesired functions If as a result of the analysis the reverse current path is declared to not be a sneak path the path will be automatically marked for deletion 4 5 9 Return to the main menu The sneak path search should be repeated for all applicable source sink pairs These include each instance of the following combinations SOURCE SINK or DC power input corresponding DC re
35. area of investigation for CAD integration is a design data base comprising parametric as well as schematic information for devices This data would expand the SCA knowledge base and thus reduce the number of user queries An evaluation of expert system shells that could best utilize this database would also be required 5 2 Expansion and Integration of the Knowledge Base One of the reasons for selecting an expert system over conventional programs for implementing the automated system was the relative ease for adding design concern rules Due to the limited scope of the current study many rules were not included Further study should not only address the addition of more rules and its resulting effect on system performance but also the possibility of including these particularly for the case of device oriented design concerns as part of already existing design rule checkers that have been developed for most schematic capture products Since many of the device oriented rules are only tenuously related to sneak circuits it may be more effective to include them with a general set of design rules available to the designer after schematic capture 43 5 3 Net List Format To permit compatibility with a wide range of CAD products SCAT was designed to process net lists formatted in EDIF At the time SCAT was developed the only available version of EDIF was EIA Interim Standard No 44 version 1 1 0 Subsequently version 2 O O has become available a
36. arly in the development cycle with no prior SCA experience required of the analyst These features are attained in part by targeting the analysis to identify sneak paths in switching circuits and commonly encountered design concerns related to sneak paths in analog or digital circuits and in part by focusing the analysis at the assembly or subsystem level rather than the entire system In this manner most sneak problems can be identified and corrected by the responsible design engineer in a timely manner The automated analysis is based in part on the fact that sneak paths involve circuit branches that conduct current in either direction depending upon the switching state of the circuit Thus SCAT searches for these bidirectional branches rather than perform the more complex task of searching for specific topological circuit patterns as done by conventional automated SCA approaches The analyst s task is also reduced to evaluating the significance of specific sneak paths rather than applying clue lists to circuit patterns for identifying the sneak paths Sneak software analysis is not addressed here 45 Since only a portion of a system is being analyzed at any given time a feature has been provided to identify sneak paths associated with assembly or subsystem interfaces SCAT requests the user to identify each interface port of a switching circuit in terms of it being a power input ground return or signal I O Interfaces to power and g
37. ate of the art and practice of the analysis technique Approximately 42 organizations were identified as prospective SCA users and contacted regarding our request for completing a survey questionnaire The organizations are listed in Table 2 2 As indicated by the first column in the table these were either prime contractors or government organizations As indicated by the last column in the table most declined to participate either because they in fact did not use or perform SCA half of the prime contractors contacted were SCA users or did not wish to divulge the requested information Seven did respond and a summary of the data they provided appears in Table 2 3 10 AAMS VOS au 103 P9398JU0 suoyezuesiQ Z Z AQEL EE A A ATA ee AA ST ER ECH MMC EC vo mega nen S70 sax soz sor IC EE E ei PU NOM 9b1095 d o gt E gt n o pon eT TA a Puan erno1 vo 7SpURbSs Ta SST BOK X5 SPprpIQ33oN ctm NACL NEN EECH A E oN Sal WI SUPITIO M N PEA EPA AE 987 ICI A EE FOR Bex wosqeser vor Meet Ss eez eor uosxser suoxAi eppes T3 o o gt ee ON LU Xi q330M q3193 q310M 3103 9877 Sex ee CE marom 3303 ENNCHIE EZ aos aos EE f ON sex Ee e reene geet uoTSTATd JTeAUOS RR CR RE CT v5 obaya ues vorerara regen Po ON 58k SNWIUOIDH uyor VO obojg ues
38. ating the SCA SCA overview data provided in a concise table format provides the PA with a means to identify areas of concern The PA will find that the resultant SCA data support design reviews and audits as well as system status determination at DD form 250 sign off Block 7 Application Interrelationship The rewrite of Block 7 provides the PA with additional guidance for applying this DID It also points out how the additional data may be used i e determine cost effectiveness Block_10 Preparation Instructions Block 10 was rewritten to incorporate requirements for those significant data elements which may be used by the PA to monitor and track sneak reports measure SCA effectiveness and determine SCA cost effectiveness 4 3 1 Modifications to DI R 7080 Ref Appendix C MIL STD 785B Task 103 requires the contractor to present an overview of the reliability program at designated program reviews DI R 7080 is the applicable DID for delivering data under Task 103 By incorporating the proposed modifications to DI R 7080 the PA will obtain reliability data which not only support existing system acquisitions but which 37 will also contribute to a database to support future system acquisitions These data will specifically support comparative analyses of analytical tx us and tests including SCA applied to program acquisitions Since analyses and tests are better suited to some applications than others gathering data on anal
39. awn in a logically consistent manner power flow from top to bottom signal flow from left to right The trees are carefully annotated to facilitate cross referencing with each other and with the analysis input data schematics wire lists parts lists etc Proposals have been made for utilizing the network tree database to support other analyses such as FMEA fault tree analysis and power loading analysis that require evaluation of circuit topology CLAR80 NP3634 RANK70 Network trees are difficult to implement because of the complex processing required for their generation Proprietary automated algorithms are used for partitioning the circuitry into segments on which the trees are based This approach may be the only one feasible for thoroughly analyzing an entire set of schematics associated with medium to large systems These drawings are usually not available until late in the development cycle i e toward the end of Full Scale Engineering Development FSED and beyond The analysis procedure can be simplified by considering clues that are independent of topology and therefore can be applied without the need for network trees This approach is particularly applicable to the early phases of a design when detailed circuit data required for generating trees are not available and is the basis for the manual and automated procedures described later in this report 2 2 3 Related Analyses As part of the data collection task information
40. bility of the analyst to understand the detailed operation of the circuit The analyst must 1 evaluate the operational implications of each reverse current path identified by the sneak path search and 2 respond to SCAT queries concerning circuit timing and the function of circuit components The size of the circuit must not exceed the analyst s capability to do so b The ability to capture the circuit using OrCAD SDT The circuit must be captured using OrCAD SDT device libraries and editing guidelines specified in section A 5 c The ability of SCAT to process the circuit For typical circuit topologies sneak path analysis can be performed on circuits containing up to 2 000 components while design concern analysis can be performed on circuits containing up to 300 components 46 If circuit partitioning is required minimize the number of interfaces crossing a partition boundary see Figure A 1 This can usually be achieved by functionally partitioning the circuitry As before all interfaces must be defined in terms of being dedicated power or ground switched power or ground or signal lines 3 Generate the EDIF net list following the procedure described in section A 5 4 Run SCAT as described in section A 6 A 3 SYSTEM REQUIREMENTS AND INSTALLATION PROCEDURE SCAT is a menu driven system consisting of 1 a set of programs written in the C language and 2 knowledge bases developed under the M 1 Teknowledge Inc expert
41. concern For the sneak path analysis the user must confirm that suspicious paths identified by SCAT are in fact sneak circuits i e they inhibit desired functions or cause undesired outputs The expert system provides assistance for this task For the design concern analysis the user must respond to prompts regarding technical details of the circuit under analysis Assistance for this is available in the form of help messages Explanations and possible solutions for identified design concerns are also available Operational details program limitations and an example of the procedure are provided by the user s manual in an Appendix A of this report A general description of the major tasks involved in the procedure is presented next CAD USER SCHEMAT IC INTERFACE CAPTURE OrCAD SNEAK NET LIST READER PATH C CODE Figure 3 1 Computer Aided System for Sneak Analysis 22 3 2 1 Schematic Capture Net List Generation The automated procedure requires that the circuit under analysis be captured and formatted by OrCAD SDT UL A schematic is captured by drawing it on the screen using the various graphics and text editing features and the device symbol libraries provided by the program All interfaces involving power and ground whether direct from external sources or from external switches drivers must be labeled using the OrCAD module port function an option provided by the program for designating signal terminations T
42. cope of SCA has been expanded to include clues for identification of design concerns in analog and digital circuitry Some design concerns imply the existence of a sneak path or sneak timing while others are completely unrelated to sneak conditions and merely indicate a violation of good design practice Design concern clues aid the analyst to identify potential problems affecting specific devices or circuit functions EMERGENCY DOOR OPEN CARGO DOOR GEAR DOWN E NORMAL DOOR OPEN ORIGINAL CIRCUIT EMERGENCY NORMAL DOOR OPEN DOOR OPEN b NETWORK TREE EMERGENCY DOOR OPEN i CARGO DOOR NORMAL GEAR DOWN DOOR OPEN EH c REVISED CIRCUIT Figure 2 1 Example of a Sneak Circuit SCA is a highly labor intensive task requiring significant computer resources for support For this reason it is typically applied only to mission or safety critical areas of a system The circuit interconnection data for these sub systems can be quite complex with documentation spread over many drawings e g circuit card schematics inter card wiring lists and subsystem cabling diagrams Automated techniques for capturing the circuitry and generating network tree interconnection data have been developed and have proved to be indispensable for efficient accurate and thorough analysis of large systems The software for performing the circuit data capture and tree generation is considered highly proprietary by those contractor
43. cts which contain the requirement for reliability Program Reviews in accordance with MIL SiD 7853 Task 103 REF EP E CEL Maier 0 Cited I hueca 10 This DID supersedes DI R 1731 and DI R 2119 MIL S10 7853 Task 103 CIL Mime Kae AMSC No F3104 OB EXEMPT MM PREPARATION INSTRUCTIONS 1 Each report shall include the following information as a minimum a The work accomplished and resulcs obtained on each task defined by the work statement or the Contractor s Reliab liry Program Plan b Summaries of the status of previously reported programs which vere unresolvec at the close of che last reporting per od c A l st of current problezs containing 1 A serial number assigned to identify the problem 2 The date on which the problem vas first detected 3 A short statement identifying the problem and its effect 4 The activity assigmed to vork on the problem S The expected resolution and date to be achieved 6 A short statement of accomplishment co date or a cross reference to other reporcs 7 The date the problem vas resolved i L MIRNA IE so oct 14 DD 33 1664 0941 ana ae Exhibit C 1 DI R 7080 Page 1 of 2 84 DI A 7080 lock 10 PREPARATION INSTRUCTIONS Continued d A specific accounting of each design review action item remaining open at the end of the last report period including a full description of the action taken on each iten e Identification of observed
44. cutc Analysts provides documentation from which che Covernmenc procuring activicy can make determin ations concerning system and equipment unwanted functions D een 0 Cd Mm asiory o cited i ot inh btcion of desired functions in che absence of component failure HIL STD 78 8 Task 203 muse be ciced n conjunction with che use of this DID MIL STO 7858 Task 205 This OID supersedes DlsR 22594 en uo ca AB EXOT AMSC Mo F3104 9 ORE AAATION 3T 4u ibn The Sneak C rcuic Analysis shall include che following daca 1 Descripcion of che mechodology and procedures used co satisfy che require ments for Sneak Cireute Analysts as sctpulaced ln HIL STO 7858 Task 203 2 Resules of che analysts and corrective actions taken or anticipated n sufficient decatl co demonscrace chac che sneak pach vill be eliminated Analyses shall be tn the contractor s own format DD Fora 1664 Replaces DSA Form 402 Which is Obsolete Page of Page U S Covernment Printing Office 1981 703 022 90 Exhibit B 1 DI R 7083 19 DATA ITEM DESCRIPTION 1 TITLE SNEAK CIRCUIT ANALYSIS REPORT 3 DESCRIPTION PURPOSE Sneak Circuit Analysis SCA documents the results of analysis performed on hardware and software systems to identify designed in conditions that could inhibit or produce undesired system OFFICE OF PRIMARY RESPONSIBILITY functions which could adversely affect crew safety mission USAF success or caus
45. d Databases File Name SCAT BAT DESIGN DEVICE DSGNMENU EDIF2MI EXE FUNCTION GOODPART PTH MI EXE MODELS SCA PATHS EXE SCA CFG SCA EXE SCAFILE SCAFILEB SCAIN SCAMENU SCAOUT File Type Size DOS batch file 1K KBase 12 3K KBase 24 4K KBase 5 9K C Program 19 5K KBase 46 1K ASCII 4K MI shell 231 6K ASCIIBK C Program 33 9K MI shell 24K C Program 36K KBase 3 2K KBase 3 6K KBase 14 6K KBase 11 4K KBase 132K Description Controls overall program flow Invoked by user Identifies power ground paths Calls PATHS Called by SCAMENU Identifies device related design concerns from the net list Called by DSGNMENU Function device design concerns selection Called by SCAT Calls FUNCTION or DEVICE Generates DEV SCA and JOINS SCA from the net list Called by SCAT Identifies function related design concerns from the net list Called by DSGNMENU Data base used by PATHS Expert system inference engine Data base used by SCA and PATHS Contains models for circuit devices Identifies power power sneak paths Called by SCAT Display configuration data Identifies sneak paths from the net list Deleted parts and source sink nodes are read from DATA SCA Called by SCAT Net list file selection Called by SCAT Reports net list file selection errors Called by SCAT Generates DATA SCA Reports presence of IC s in the net list Called by SCAMENU Sneak paths design concerns
46. d Fail Safe Technology Corporation as subcontractor for the Rome Air Development Center Griffiss AFB NY under contract F30602 87 C 0193 Technical direction for the study has been provided by Mr Bruce Dudley The author wishes to acknowledge the outstanding effort of J Agron and H Baik of SoHaR for developing respectively the C code and M 1 knowledge base rules for the SCAT system The author also wishes to acknowledge the contributions of K Smernoff of Fail Safe Technology for performing the SCA user survey and J Sivak and D Friedman of Fail Safe Technology for performing the SCA control and monitoring study Finally the author wishes to thank B Dudley of RADC and H Hecht of SoHaR for their technical guidance and P Crane of SoHaR for assisting in the text and graphics processing required for this report NTIS TRAGI v OTIC TAB a Unannoi ced o Justification tM Distr ibution Availability Codes Avail audor Special ul Dist TABLE OF CONTENTS EXECUTIVE SUMMARY lt a dc vi I INTRODUCTION ceee DAA EE Y Eh eh 1 2 BACKGROUND 6 4933 vrbs re ws iue cS E Vs PD 3 2 1 Conventional Teclinqu s acea ox SRA A AA 3 22 Intenni RESUS al oues RE AEN vos em doe a eed De zd 6 22 1 Literature Search and Analysis lt o oooooo oso 6 2 2 2 US GUEVEY A a Ro SOLET E Ee A 10 22 4 Related AUS AA eae e WE x Go QUA en 13 2 3 A Simplified Manual Procedure pes E ER E EA ES
47. d through the Gear Down contactor A secondary switch permits emergency operation of the door when the gear is not down Due to a sneak path closure of the emergency door switch when the primary switch is closed will inadvertently lower the landing gear In the conventional SCA approach accurate production level drawings of the circuitry are required to insure all circuit paths are considered by the analysis The circuit interconnection data are partitioned for constructing network trees to filter non relevant schematic data and generate a visually simplified presentation of the circuit Several versions of the trees may be required to analyze circuit switching configurations corresponding to a timed sequence of system states The topology of each network tree is analyzed for the appearance of the key patterns for the cargo door example an H pattern is recognized The H pattern is more apparent from the network tree drawn in part B of Figure 2 1 than from the circuit schematic drawn in part A The tree is constructed by tracing all possible paths from power to ground assuming all switches are closed and is drawn such that power flows from top to bottom Appropriate topologically oriented sneak clues are then applied to the pattern and if an answer is affirmative the sneak path is identified In this example it can be prevented by insertion of a diode in series with the primary switch as shown in part C of the figure In recent years the s
48. ding interruption of power at the collector terminal of specific transistors will appear Select q3 then select q2 A warning regarding a possible sneak path through the transistor will appear The choices explanation solution warning message continue and return may be selected as before Return to the design concern type menu by either selecting return where available as a choice pressing ALT A or repeatedly answering queries and selecting continue until the analysis concludes and then selecting return At the design concern type menu select return The main menu will appear this point the basic example concludes Select exit program to exit to DOS Otherwise proceed to step 26 to continue this example 26 At the main menu Figure A 8 select model switch The switch relay model menu Figure A 20 will appear Select append M A prompt regarding specifying specific switches as Make Before Break will appear 62 27 Enter lt s3 gt in response to the prompt Figure A 21 Note the switch entry displayed in the DISPLAY window Enter k1 Enter done to return to the main menu Again note the switch entry appearing within the message in the DISPLAY window Figure A 22 28 At the main menu select sneak The IC message will appear 29 At the IC message select continue The sneak input data menu will appear 30 At the sneak input data menu select execute The system wil
49. e branches of the pattern form an X such that power flows into the branches at the top and out the branches at the bottom A topological pattern within a network tree The branches of the pattern form a Y such that power flows into the branches at the top and out the branch at the bottom 93 IIA EI EE EM ASI RI Qe OE RI RIE RAS LINA AAA LA EAD SE e AD MIS RAID OF u 39 A EI MIF AEN 90 EE FOE KE AK Q0 DE HOE KD HKD MISSION E of Rome Atr Development Center RADC plans and executes research development test and selecied acquisition programs in subport of Command Control Communications and Intelligence C I activities Technical and engineering suppert within areas of competence is provided to ESD Program Oyfices PO and other ESD elements to perform effective acquisition of C I systems The areas of technical competence include communications command and control battle management information processing surveillance sensors intelligence data collection and handling solid state sciences electromagnetics and propagation and electronic reliability maintainability and compatibility Xs 9 xS v IN OFF ARIF ALF AFR PLS PIF AIF PSF Je xo HH e
50. e circuit consist of analog and electromech devices only digital IC s amp passive devices only both analog and digital devices NSWER analog digital unknoun Space to Mark ALT A Abort F2 Scroll Display F10 Command Menu Modify if necessary Figure A 17 Circuit Type Menu External Power Source ac_pur ac_rtn src x_plusSudc Ground snk The circuit consists of Both analog and digital devices Continue Modify External Pouer List Modify Ground List Hodify Circuit Type Return to Menu 71 NSUER DIS E pouer list gnd list circuit Lupp return unknoun Space to Mark ALT A Abort F2 Scroll Display F10 Command Menu Figure A 18 Design Parameter Summary Mi Commands 7 PPLICATION DISPLAY The following topics are covered under FUNCTIONAL guidelines Power distribution and power switching Digital timing status indicators i e lamps LEDs The follouing topics are covered under DEVICE guidelines Bipolar transistors Op amps Noise sensitive circuits op amps one shots SCRs Relays TTL or MOS digital circuitry UES TION NSWER Choose desired operation return functional_guideln return to main menu evice_guideln functional guideline unknown device guideline pace to Mark ALT Abort F2 Scroll Display F10 Command Menu Figure A 19 Design Concern Type Menu M1 Commands gt PPLICATION DI
51. e e A A A A A A A A A A A A A A A a APPENDIX A SCAT USER S MANUAL TEE 45 APPENDIX B PROPOSED REVISIONS TO DR 78 APPENDIX C PROPOSED REVISIONS TO DE R 7080 ooo oooo o 83 REFERENCES 0 0 0 0 cece cece eser hehe 88 ANNOTATED BIBLIOGRAPHY eene a rer rere 89 e ere Een 92 iil Ee 9 LIST OF FIGURES Figure 1 Sneak Path Identified by the Automated Pfocedtre viii Figure 2 1 Example of a Sneak Circuit o o oopurroomon ororo oro r o s 5 Figure 2 2 Design Rule Example um LAS PARA 18 Figure 2 3 Guideline EE A RN 19 Figure 3 1 Computer Aided System for Sneak Analysis 22 Figure 3 2 Reverse Current Path in Missile Launch Circuit 25 Figure 3 3 SCAT Reverse Current Path Display 25 Figure 3 4 Power Tie in a Weapon Station Circuit 2 ee ee ee ee 26 Figure 35 SCAT Power Tie DISplay 2229 acid eased el OA EN 27 Figure 3 6 Explanation and Solution Messages o oo 21 Figure 3 7 Transistor Reverse Current Sneak ooooooo ooo ro ooo 29 Figure 3 8 Transistor Sneak Display o oooooo oo 29 Ele 3 0 Tesh Case LS A US Re a 31 Figure 3 10 Test CASETE ADS ae AA 32 Figure 3 11 Test Case HI a AS A eA eect 33 Figure 4 1 Vehicle or Application System File 39 Figure 4 2 Analysis and Test Data File lt lt 40 Figure A 1 Proper Circuit Partitioni
52. e equipment damage This report details the meng analysis methodology and the status of each sneak problem 6 DOC REQUIRED identified as a result of SCA It provides insight into the extent and depth of the SCA APPROVAL LIMITATION APPLICATION INTERRELATIONSMIP 7 1 This SCA Report provides data which may be used by the procuring activity for statistical analysis and further to determine SCA cost effectiveness HI TENEN CES Mandatory as cited in oc 2 2 This Data Item Description DID contains the format MIL STD 785B Task 205 and content preparation instructions for the data product generated by the specific and discrete task requirement as delineated in the contract 7 3 This DID 1s applicable when a Sneak Circuit Analysis is required and performed in accordance with MIL STD 785B Task 205 WSL NUMBER S 7 4 This DID may be applied to any contract during the 10 PREPARATION INSTRUCTIONS 10 1 Format Contractor s format is acceptable 10 2 Content The SCA Report shall include the following data 10 2 1 Methodology gt description of the methodology and procedures used to satisfy the requirements for SCA as stipulated in MIL STD 785B Task 205 Specify computer resources and test equipment used to perform SCA 10 2 2 Summary A program summary of the results of SCA Include problems encountered accomplishing the analysis and program elements procedures or analytical techniques which aided accomplishing the
53. ection for attempting this involves the MS DOS operating system for the PC The user Product B in Table 2 1 provides an open database and ASCII file formats 8 euroJgoououl 2 Io Oo g IMOMOV SIL TU JA OQoOomommuooumnunmnunmonmnon E OSPIA X Cx X X X X X X X x X x X X X x X X X X X X X X x X X X 60 O0 X X X gt de X X X X X ur jo D 00 ony E xy uy os X X X BON JIQH JO OO So1 ge m m uondo axy a 000 ayy 000v ayy O00 axy 008 a OOF a Oo ayy 056 OXW oos qe v 000c a 006c Qv OTI Qv O00I OXW 0007 ayy 000 GV 004 ERI em Jqrj 1QPI S6v 66 S6r S6Vr 69 0S9 059 68 S66 S6 OSSE S6v 001 S6 s6v S6b Ls 000 oos 000 T SE 9861 861 9861 v861 9861 S861 S861 861 9861 9861 86T C861 9861 1361 9861 s861 Our w sprpoig o4njdv DIJBULSUYIS poseg Dd JO LIAMNS 8861 T Z AQEL Gi 30 SUOTIPIAOI tonne e Nsomucted JON On Nu AL em D nf ul aHa Du ula won ws wu uda KO LASs avoJO jonpoug would have to manually perform the time consuming steps of exiting and re entering the programs each time he desired to view the SCA results on the schematic The possibility of adding sneak circuit checklists to CAD schematic capture products was also investigated Most schematic capture products include a rule checking capability for identifying certain types of drawing erro
54. ed sneaks must be validated and tracked by the contractor to disposition Some examples of sneak disposition include no action system redesign or follow on modification Once verified sneak reports gain increased visibility Those with severe safety or equipment damage impacts are most visible and evoke PA involvement 4 2 Recommended Procedures 42 1 SCA Follow up The SOW should require the contractor to track all identified sneaks through to disposition and to provide resultant data to the PA In order to maintain proper system baseline control the contractor may present the identified sneaks to various review panels and boards The PA should attend even the informal reviews in order to monitor progress and to provide the customer viewpoint when appropriate The status of all identified sneaks should be required at key program milestones for the PA to assess overall reliability program progress Recommended milestones are Critical Design Review CDR Functional Configuration Audit FCA and DD Form 250 sign off 4 2 2 SCA Effectiveness This study considers SCA effectiveness to be a measure of the number of identified sneaks related to the number of resultant design changes implemented In order to analyze and measure this effectiveness on one program or across many programs data must be available which support such an analysis It should be noted that relevant SCA data collection was also recommended in a study completed in 1982 by Boeing Aero
55. ent and for maintenance and logistics activities It is thus a primary task under MIL STD 785B that should not be made dependent on the results of SCA Safety analyses include the preliminary hazard analysis for evaluating potential hazards early in the system design system hazard analysis for identifying hazards later during system development operation hazard analysis addressing hazards associated with fielded equipment fault hazard analysis for identifying potential hazards caused by component failures and accident analysis The sharing of data and techniques with safety related activities may be deliberately restricted in order to keep safety activities for highly critical equipment independent of reliability and design However in other cases information sharing may be permissible and should then be encouraged in order to contain costs 23 A Simplified Manual Procedure The manual SCA procedure developed during this study is a simplified version of the conventional procedure It is intended for use by the design engineer or reliability analyst as a means for both avoiding designs likely to contain sneak paths and for identifying most instances where sneak paths exist It is not intended as a substitute for the conventional procedure applied to a fully developed system but instead serves as a simplified method for minimizing the number occurrences of sneak conditions early in the design effort 16 The procedure was developed by
56. er change An evaluation of the two reverse current paths identified reveals the critical one to be Path 1 The path permits source current flowing through relay K1 when de energized and through switch S2 when enabled to also flow through the coil of K1 thus energizing the relay This in turn will open the K1 contact de energizing the coil and starting an oscillatory sequence of events Note that current through S2 flows opposite to that implied by Path 2 Q1 S2 LP2 see Figures A 13 and A 14 Hence S2 is bi directional and is prefaced by an asterisk Select return The main menu will appear 8 At the main menu select design The power source menu Figure A 15 will appear 9 At the power source menu select continue The ground list menu Figure A 16 will appear 10 At the ground list menu select continue The circuit type menu Figure A 17 will appear 11 Answer the circuit type query both The summary of design parameters Figure A 18 will be displayed along with options to modify any of them 12 Select continue The design concern type menu Figure A 19 will appear 13 At the design concern type menu select functional guideln The function oriented design concern analysis will commence 14 A query will appear regarding power and ground connectors Select no 15 A warning regarding a possible shock hazard at the ground pin will appear Select explanation An explanation of the concern will appe
57. erance design compliance analysis has an important interface with SCA because the criticality of the functions that make SCA necessary also require fault tolerance and the latter must address the absence of design hazards such as sneak circuits that can defeat its purpose The manual SCA procedure was documented in a separate report entitled Sneak Circuit Analysis for the Common Man report number RADC TR 89 223 The procedure is intended for the design engineer or electronics reliability analyst without prior SCA experience The report includes a simplified set of sneak related design concern clues that can be applied to circuitry at the assembly or subsystem levels by personnel who understand the operation of the circuitry and the devices comprising it Supplementary explanations problem illustrations and recommended solutions are also provided The clues address areas such as improper switching elements in power return lines timing problems associated with relay circuits and problems associated with application and removal of power to digital circuitry The report can also serve as a guidebook for familiarizing engineers with the techniques for designing circuits free of commonly encountered sneak problems An SCA tool SCAT was developed to automate the manual procedure and to extend it to include automatic identification of sneak paths in switching circuits An example of a sneak path identified by SCAT is shown in Figure 1 The highlighted
58. ersonnel they are time consuming and tiring to use because they Lack structure by not being arranged in a manner that permits skipping a number of subordinated items when a negative no sneak circuit possible finding is reached for a top level clue Mix areas in which subjective analysis is required such as the appropriateness of labels with areas in which clear decisions are possible such as the possibility of unwanted current flow Include questions that are clearly the responsibility of the design engineer such as the compatibility of loads with power sources Information on problems related to the performance of SCA has been amply reported in the literature BALD87 BURA82 and are evident in final reports of specific SCA applications The major problem areas arise from performing SCA too late in the development cycle by a SCA specialty team too removed from the design effort and from the diversity of interests of the performing organization with the organization responsible for the design Thus the results of a thorough analysis are typically contested by the design organization either because the sneak circuits identified in fact do not pose a problem or because their degree of significance does not justify the cost of their removal The solution is to simplify the analysis such that it can be applied in the early phases of the design effort either by design personnel or under their guidance The purpose of the CAD surve
59. es of a development effort The extent of the investigation of schematic capture products performed during this study is shown in Table 2 1 Data for the table were compiled in August of 1988 In addition to basic product information year product was introduced number of sales and cost the table also includes the following technical characteristics Library size The number of unique graphical component symbols available for schematic editing Library type Type of components covered analog A digital D or both EDIF Net List Compatibility with the EDIF industry data formatting standard Layout Tool Product can also perform component layout Analysis Tool Product can also perform circuit or logic analysis External Annotation Capability for external programs to modify the schematic Hierarchical Schematics Capability for representing schematic data hierarchically in addition to a flat multi page format Rule Checking Provisions for identifying violation of design rules Video Compatible with high resolution color monitors C high resolution monochrome monitors M or both B Based upon this investigation the possibility of displaying SCA results within the on screen schematic was considered impractical because none of the products surveyed provided a means for graphically highlighting a specific component or path and except for one product hooks were not provided in their software for a user to do so An additional obj
60. es of this report unless contractual ebligations or notices on a specific document require that it be returned Form Approved OPM No 0704 0188 d DEE indudeg FUCBONS Re exin dem SSU ON getecng and manus te Xa nacoo APO ECO Me oolsonon ot F ermason Send commen fe roen cci mae or any ara moat Ze col ecson of Inurmaten POLONO LUGON son for recuang Ce Droen p Wactsngion Heacquaners Servces Deeg mae Sei Repors 1215 Jetersen Dave Hignersy Saw 1204 Agen V 22202 4202 aon Management ans Suet Yestergon DC 20603 tre Ome of Nirmeen and Regsewry At Olka of 2 REPORT DATE 3 REPORT TYPE AND DATES COVERED June 1990 Final Oct 87 to Oct 89 1 AGENCY USE ONLY Leave Blank 5 FUNDING NUMBERS 4 TITLE AND SUBTITLE INTEGRATION OF SNEAK ANALYSIS WITH DESIGN C F30602 87 C 0193 PE 62702F PR 2338 6 AUTHOR S TA 02 WU 3R Jeff Miller NIZATION 7 PERFORMING ORGANIZATION NAME S AND ADDRESS ES 8 Ni ORGA ZA SoHaR Incorporated 1040 South LaJolla Ave Los Angeles CA 90035 2525 9 SPONSORING MONTORING AGENCY NAME S AND ADDRESS ES 10 SPONSORINGMONITORING AGENCY REPORT NUMBER Rome Air Development Center RBER RADC TR 90 109 Vol I Griffiss AFB NY 13441 5700 of two 11 SUPPLEMENTARY NOTES RADC Project Engineer Bruce W Dudley RBER 315 330 2608 12a DISTRIBUTION AVAILABILITY STATEMENT 12 OISTRIBUTION CODE
61. everse Current Path 1 75 Figure A 26 Deleted Paths Screen ei rs a a E 75 Figure A 27 Power to Power Tie Warning Message o o oooo oo ooo 76 lv A M um Figure A 28 Power to Power Tie Highlighted on Schematic 76 Figure A 29 SCAT Program Flow ciue does daca RD PORC p RN da 7 LIST OF TABLES Table 2 1 1988 Survey of PC Based Schematic Capture Products 9 Table 2 2 Organizatioris Contacted for the SCA Survey ss 11 Table 2 3 SCA Survey Summary lt lt ocd cheng me ce OX ORE RS 12 Table 2 4 Equipment Concerns of Reliability Analyses 14 Table 2 5 SCA Interfaces with Reliability Analyses LL 15 Table 3 1 Test Results ou id A a 31 Table 4 1 SCA Report Checklist cromos corr 42 Table A 1 SCAT Programs Knowledge Bases and Databases 49 Table A 27 SCAT Data Files e erg peo AS RA ew 50 LIST OF ACRONYMS CAD Computer aided design CAE Computer aided engineering CDR Critical Design Review DCA Design Concern Analysis DID Data Items Description EDIF Electronic Data Interchange Format EIA Electronic industries Association FCA Functional Configuration Audit FMEA Failure Modes and Effects Analysis FSCA Functional SCA FSED Full Scale Engineering Development IO Input output PA Program Authority PC Personal computer SCA Sneak circuit analysis
62. eview and final submittal prior to Functional Configuration Audit 7 6 The Contract Data Requirements List CDRL should specify whether this document is to be prepared and delivered on bound 8 1 2 x 11 inch bond paper or electronic media 1f electronic media is selected the precise format must be specified This DID superseded DI R 7083 BLOCK 10 CONTINUED operating conditions and thus may result n an undesired action taken by an operator 4 Sneak Label A label which incorrectly or imprecisely labels system functions e g system inputs controls displays buses etc and thus mislead an operator F Nature of the Sneak Identify the causal nature of the sneak as 1 Specification Error 2 Design Error 3 Manufacturing Error 4 Other Sneak Severity Category Identify sneak category as 1 Category 1 Catastrophic May cause death or weapon system loss 2 Category II Critical May cause severe injury major property damage or major system damage which wil result in mission loss 3 Category 111 Marginal May cause minor injury minor property damage or minor system damage which will result in delay or loss of availability or mission degradation 4 Category IV Minor Not serious enough to cause injury property darage or system damage but will result in unscheduled maintenance or repair H Disposition Indicate if a change was implemented and implementation details 1f a change was not imple
63. form the SCA and 2 total program cost Additional comments such as the size of the system under acquisition should also be included Ideally the information would be compiled in a relational database to facilitate retrieval for cost effectiveness analysis A relational database is recommended since it provides the required link between significant data files such as contractor analysis size and cost SCA cost effectiveness addresses the relationship between the cost of performing SCA costs associated with correcting sneak problems and costs associated with other reliability analyses such as FMEA Fault Tree Worst Case Finite State Machine e g analyses utilizing Markov models or Petri net diagrams which seek to identify deficiencies prior to integration and test Cost data for these other analyses should be provided by the PA for inclusion in a database The dati re used to perform the following analyses l Compare the cost of the SCA with the cost of removing the sneak circuit by other means 2 Compare the cost of the SCA with the cost of the failure that would occur if the sneak circuit was not found 4 2 4 SCA Thoroughness SCA thoroughness is a measure of the type and quantity of sneaks identified as a result of analyses other than SCA This measure is appropriate when other analyses are performed 36 on a system whether or not a SCA is performed Other analyses might include Failure Mode Effects Analysis Fault Tree A
64. gements single and multiple pole throw break before make and make before break Transistors both bipolar and MOS and diodes Capacitors under conditions of both AC and DC current flow Other two terminal passive devices resistors inductors etc Multi terminal passive devices transformers potentiometers etc The user may override either globally or for specific devices the default model assumed for switch and relay contact timing break before make and for capacitors open circuit The more recent version 2 0 0 was not available in time for this effort 23 Connections to integrated circuits are modeled as open circuits since paths are not traced through these devices Following the path search path sets are identified in which a common branch conducts current in both directions These bidirectional branches are usually indicative of an undesired reverse current path the distinguishing feature of a sneak circuit Each reverse current path is displayed as a list of schematic reference designators of the devices that appear along the path listed in the order of their appearance between the source node and sink node Once identified the user must confirm the validity of the path by considering operational constraints that may preclude certain switching states assumed by the path in question In addition their potential impact on system operation must be evaluated The SCAT expert system provides guida
65. h execute command rather than SCAT startup so as to not include the length of the user interactive startup session 28 Signal Generator Figure 3 7 Transistor Reverse Current Sneak Mi Commands PPLICATION DISPLAY WARNING There may be a sneak path from the base of the transistor s q1 through the forward biased base collector junction to the collector load NSWER Choose one of the following options explanation solution warning message return unknown Space to Mark ALT A Aburt F2 Scroll Display F10 Command Menu READY Figure 3 8 Transistor Sneak Display 29 Results for three test cases appear in Table 3 1 Case 1 Figure 3 9 was extracted from an actual application of avionics switching circuitry in order to test a representative circuit topography i e realistic ratios of nodes to branches and of series branches to parallel branches Cases 2 and 3 Figures 3 10 and 3 11 were created by respectively doubling and quadrupling Case 1 and adding a bidirectional branch where before there was none The net lists corresponding to these cases were analyzed by running SCAT on two different host computers an 80286 based and an 80386 based machine configured without math co processors The resulting path search times ranged from 15 seconds smallest circuit fastest computer to 85 seconds largest circuit slowest computer The procedure s effectiveness as defined by the first two points in
66. he most critical parts of SCA particularly for relay circuits and coordination of the activities presents an opportunity for substantial cost savings The analysis addresses the 14 svo BONUS jo uonvonnuop sjuouoduroo eonuo jo uomneormuop sjuouoduroo tono jo uoneoynuopy seo Bono jo uoneornuop YIHLO Sed yesus punolo su1oouoo ugIsop poiuotlo DILADP AMOS quowdinba po tej uou jo uonelodo popuaiutun Jo sodoidunt jo 1093 4 pornuopi ouoN dV TidAO uonnqmsip punoip oseq vep 1uouodulo sure1derp guru suioouoo UZIS soseq wep uouoduos Sopou IN JIOMION sulo9uoo UZIS ampez Woy Sun nso1 suonipuop DINE o Surpeo SuonIpuo SNIJVHS V LVG fAouro1 quouidinbo oi anp spuno13 Surpo p 1dooXg y uonelouod oan JION stsKpeue Surum 31307 StsKpeue 3191 siskjeue Surum 91307 siskyeue yo uonelousd HN XIOMON ONRIVHS TOOL sashjeuy APNE YA S9IBJ1UL V S ez ALL Sutpuno15 DOUBIIIOL sueg 2 AilAnisuss asep ISIOM DOLL Ney IdAL SISATVNV 15 possibility of current flow between ground nodes a condition often associated with sneak circuits Power and load analysis another design technique eyaluates open circuit voltages and short circuit currents on lines controlling hazardous functions With regard to the failed equipment analyses the most important interfaces exist with fault tree and failure modes and effects analysis Fault tree analysis is a
67. hed variables a facility for rule looping and a large number of built in meta facts and meta propositions and its open knowledge base the rules are formatted in ASCII text The SCAT knowledge base consists of approximately 265 rules stored in nine data files The knowledge base is augmented by approximately 650 lines of C source code implementing those portions of the analysis requiring intensive processing These portions include reading the EDIF net list and performing the sneak path search The code was developed on Microsoft s QuickC environment and C 5 0 Optimizing Compiler Information needed for maintaining the program including extensively documented source code and knowledge base files appears in Volume II of this report To maintain an audit trail each file is prefaced by a header identifying the date of the last revision its originator and a brief description of it 21 A functional diagram of the system appears in Figure 3 1 Prior to running SCAT the user must generate a net list of the circuit to be analyzed using the OrCAD schematic capture program The net list must be saved either on hard disk or floppy disk SCAT reads and processes the net list as directed by the user to identify sneak paths and design concerns Utilizing the user friendly consultation type interface provided by the SCAT expert system the user must specify the name of the net list file and the type of analysis to be performed sneak path or design
68. his will enable SCAT to account for all significant interfaces to portions of the system not under analysis In addition the terminals of all internal power sources e g on board batteries must be similarly labeled to address potential sneaks involving power to power ties After capturing the schematic the net list is generated and saved using the OrCAD FlatEDIF utility When invoked by the user this utility translates the captured schematic into an ASCII text file conforming to the Electronic Industries Association EIA Interim Standard No 44 for EDIF version 1 1 0 3 22 2 Sneak Path Analysis After generating the net list the user enters SCAT and specifies the name of the net list file to be processed The user is then given the option of performing a sneak path analysis or the design concern analyses The following discussion assumes the former has been selected Sneak path analysis is performed on power switching circuitry i e circuits involving combinations of current interruption devices such as switches relays fuses connectors and transistors During the analysis all possible non cyclic i e non intersecting directed paths are automatically identified between every pair of power and power return points in the circuit herein after referred to as the source node and sink node specified by the user To facilitate this path search SCAT automatically models the following types of devices Switch and relay contact arran
69. iated with circuit topological patterns and 2 those associated with specific devices or circuit configurations Cyclic Path Any closed i e circular topological path through a circuit EDIF Electronic Data Interchange Format an industry standard governing the transfer circuit data such as electrical schematics between computer aided design tools Expert System Shell The basic software the inference engine required for processing a set of rules constituting a knowledge base application and the software facilities for developing and maintaining the knowledge base Fault Tree Diagrams employing a special logic type symbology for depicting the hierarchical dependency of higher level failure events on lower level events Finite State Used in reference to analyses utilizing Markov models or Petri net diagrams where the operation of a system can be represented by transitions between a finite number of processes or states Funtional Net A functional block diagram depicting power distribution and control and major signal flow between system functional elements H Pattern A topological pattern within a network tree The branches of the pattern form an H such that power flows into the branches at the top and out the branches at the bottom The branch represented by the cross bar of the H can potentially conduct current in both directions and therefore may be a sneak path 92 K Base File MBB Switches Make Before Brea
70. ic devices DEVICE knowledge base and 2 circuit configurations involving circuit functions such as power distribution FUNCTION knowledge base Design concerns are implemented as knowledge base rules and comprise the functional guidelines and device guidelincs appearing in the guidebook Sneak Circuit Analysis for the Common Mar When a design concern is encountered an appropriate message is 3 The guidebook s design rules for avoiding sneaks during design were not implemented during this effort because they apply functionally to the overall circuitry and are therefore much more difficult to automate 52 displayed and the user is given the options of requesting an explanation of the problem and a possible solution The analysis concludes by returning to the main menu AS SCHEMATIC AND NET LIST GENERATION Schematics must be drawn using the graphics and text editting tools and parts symbol libraries supplied by OrCAD SDT I The product includes print and plot utilities for generating hardcopies as well as a utility for generating net lists Completed schematics are saved on disk and may be retrieved for additional editting A complete description of the product can be found in the OrCAD user s manual and is not presented here Before generating a net list for input to SCAT the schematic must be checked for the following 1 All interfaces to external power sources and ground nodes must be labeled using the OrCAD SDT module port
71. intended electrical paths within a circuit and its external interfaces Sneak Timing Unexpected interruption or enabling of a signal due to switch circuit timing problems Sneak Indications Undesired activation or de activation of an indicator Sneak Labels Incorrect or ambiguous labelling of a switch Because it was found that frequently encountered causes of sneak circuits were associated with distinct topological patterns on circuit diagrams the identification of these patterns and the recording of specific circuit attributes applicable to each pattern were considered efficient means of using past experience to guide a current analysis This conventional approach led to the development of semi automated methods of isolating the topological patterns in relay circuits and to the generation of clue lists applicable to each type of topological pattern The most significant of these patterns are the Y power dome inverted Y ground dome and H cross tie where in each case the pattern depicts power flow from source s to ground s Additional patterns for analog and digital signal flow have also been developed A simple example of the conventional approach is demonstrated with the help of Figure 2 1 The functional circuit depicted in part A of the figure is intended to prevent routine opening of a cargo door unless the aircraft is on the ground For this reason the primary switch that controls the door opening is energize
72. k M 1 Net List Network Tree Non cyclic Path OrCAD Schematic Capture X Pattern Y Power Dome A knowledge base file i e a computer file containing a set of rules constituting a knowledge base See Make Before Break Refers to any switching device e g switch relay contactor having a multiple contact arrangement such that upon being switched the selected contacts will close e make before the de selected contacts open i e break The trademark of a commercially available expert system shell from Teknowledge Incorporated A textual listing of the circuit interconnections and devices appearing in a graphical schematic Various formats including EDIF are available for organizing the list A diagram depicting a small functional portion of a system s circuitry with all extraneous interconnections and devices removed so as to highlight the circuit topology The tree is drawn such that power flows from top to bottom and signals flow from left to right Elements of the tree are cross referenced to the detailed electrical schematic s from which the tree was derived A topological path through a circuit in which the path progresses without ever crossing back upon itself The trademark of a commercially available schematic capture product from OrCAD Systems Corporation The process of generating editting and saving an electrical schematic on a computer A topological pattern within a network tree Th
73. k Circuit Analysis Fault Tree Analysis Finite State Machine Analysis Failure Mode Effect Analysis burn in test integration test etc h Hazard Category if identified d A specific accounting of each design review action item remaining open at the end cf the last report period including a full description of the action taken on each item e Identification of observed potential reliability problems introduced by Government furnished equipment and descriptions of accommodations or improvement changes deened necessary to rake such equipment compatible 3 2 The report shall include a graphic discussion of trends A breakdown to the configuration item level shall be rade in the following manner Allocated Predicted Observed Value Value Value 3 The report shall include proposed changes to the Reliability Program Plan as applicable 4 The Final Status Report can be identified as the Program Summary Report Exhibit C 2 Continued Page 2 of 2 87 ATA References BALD87 V D Baldwin Long Term Storage Effects on Fuzes AFATL TR 87 03 AD B110924L 26 January 1987 BOEI75 Boeing Co Electrical Analysis of B 52 FB 111 AMAC and Release Circuitry Utilizing Sneak Circuit Analysis Techniques report D2 118576 1 2 3 AD A103951 2 3 October 1975 BURA82 D L Buratti and S G Godoy Sneak Analysis Application Guidelines RADC TR 82 179 AD A118479L June 1982 CLAR76 R C Clardy Sneak Circuit A
74. l search the net list for reverse current paths The sneak paths screen will appear 31 At the sneak paths screen observe that four reverse current paths have now been identified The additional two paths are due to relay k1 and switch s3 having been modeled as Make before Break 32 Select previous Path 4 Q1 S2 K1 S LP1 will appear Figure A 23 33 Select analyze A query regarding permissibility of simultaneous switching will appear Select no 34 The system will conclude that reverse current Path 4 does not present a sneak problem and will mark the path for deletion Figure A 24 This conclusion can be overridden by selecting the undelete option Select regenerate paths 35 The system will search the net list for reverse current paths under the constraint that the marked path is not a sneak path Observe that only two paths were found Figure A 25 The first of the four previous paths K1 S LP2 is no longer a reverse current path due to the deletion of Path 4 Select deleted paths 36 The deleted paths screen will appear Figure A 26 The options next and previous can be used to view deleted paths when more than one exists The option undelete can be selected to mark a deleted path as a valid sneak path Select paths to return to sneak paths menu 37 Select return The main menu will appear 38 At the main menu select design A summary of the currently selected design concern analysis parame
75. mented provide reason for not implementing the suggested change For example 1 False ID The sneak problem was falsely identified 2 No Problem The sneak design aid not manifest itself into a sneak probler 3 Ignored The sneak effect was operationally insignificant or would not cause functional failures 4 Time Budget Program time or budgetary constraints outweighed the risk associated with not making the change 1 CCB Date If the resultant design deficiency was presented to the Configuration Control Board provide the date J CCB Number Configuration Control Board assigned number if applicable X CCB Action Action taken upo initial presentation to Configuration Control Board L Manhours Estimate manhours required to correct sneak Exhibit B 2 Continued Page 2 of 3 81 BLOCK 10 CONTINUED M Cost Estimate cost to correct sneak N Status Date Status of sneak report Status date shall be within 1 days of DID submittal Exhibit B 2 Continued Page 3 of 3 82 Appendix C PROPOSED REVISIONS TO DI R 7080 DATA ITEM DESCRIPTION NA UTE RELIABILITY STATUS REPORT DI 2 7080 To monitor and evaluate contractor s progress and accoeplish c overcs or sanaat ments n conducting the Reliability Program for che ALAN applicable contract end itea s USAF 90 ALOLUIALO 9 amp 4999QvY4L Le TA TIO y APPLIC AT IO Ms INT EMRE ATADO rim Applicable to contra
76. ms identified with their performance The goal is to have a database linking these two files such that program data can be related to analysis and test data and their identified problem areas The significance of having two files is simply that the data sources for these files are different Program data are available at the government s program office and analysis test data are available with the contractor s reliability organization Since the contractor will likely be required to submit the analysis and text data on a regular basis this data will be available for inclusion in the database file The two database files are described below 4 4 1 Database File 1 Vehicle System Application File This file contains data pertinent to acquisition programs which may range from major weapon systems to small modifications Relevant data are program costs and milestone dates Figure 4 1 below suggests pertinent data fields within file records Detailed information about record fields in File 1 follow 38 FILE 1 VEHICLE OR APPLICAT ON SYSTEM FILE FIELD DATA VEHICLE OR APPLICATION 1 2 DESIGNATOR 3 PROGRAM COST 4 CONTRACT COST FOR EACH ANALYSIS AND TEST 5 PROGRAM MILESTONE DATE FOR DEMONSTRAT ION V ALIDATION PHASE START 6 PROGRAM MILESTONE DATE FOR FULL SCALE DEVELOPMENT PHASE START 7 PROGRAM MILESTONE DATE FOR PRODUCTION PHASE START 8 PROGRAM MILESTONE DATE FOR OC 9 PROGRAM MILESTONE DATE FOR PREL
77. n unknown Space to Mark ALT A Abort F2 Scroll Display F10 Command Menu Figure A 27 Power to Power Tie Warning Message Figure A 28 Power to Power Tie Highlighted on Schematic 76 Net List Entry A 6 Filename E Design Sneak Exit Power Source A 14 Continue Continue Ground List Sneak A 15 Input Data f A 9 Continue a us Reverse Current Path A 10 Continue Design Param ummary A 17 Retum Continue A Concem Type 8 A 18 Functional Device Next Previous Functional Retum Device Design Design Concems ms Conce Retum Retum Explanation Explanatior 8 Solutior B Solution Waming Msg Waming Msg Figure A 29 SCAT Program Flow 77 Appendix B PROPOSED REVISIONS TO DI R 7083 DATA IT DESCRIPTION sotmer e ara SNEAK CIRCUIT ANALYSIS REPORT P Dl R 7083 y Ofscaqir 90 97e9501 amp 4 A Ova Jar The Sneak Circuit Analysis documents che results of ans 80 DEC 1 lyses performed to verify che absence or presence of hidden flow paths unexpectad outputs or undesirable functions of equipment or sofevare The results of che analyses identify any latent flow pachs chat could cause unexpected operations e cece exevaem during che life of che hardware or sofcvare and corrective action proposed co eliminace them It decatle che method ology used in and che extent and depth of the analyses APPLICATOSy T CAMEL A GU The Sneak Clr
78. nalysis Common Cause Failure Analysis Sensitivity Analysis Worst Case Analysis Power and Load Analysis Grounding Analysis Finite State Machine Analysis Preliminary Hazard Analysis Desk Checking and Peer Code Review To measure SCA thoroughness in this context the same data categories are required from the analysis contractor as is noted above for SCA effectiveness 4 3 Proposed Data Items Implicit in this investigation was an implementation of the recommended procedures using the most cost effective methods possible with minimal impact on contract requirements Toward this end the preferred approach to obtain data was to modify existing DIDs in lieu of creating new DIDs The affected DIDs are DI R 7083 and DI R 7080 The DIDs and the proposed changes to them are presented in Appendices B and C A discussion of these changes is presented in the following paragraphs 4 3 1 Modifications to DI R 7083 Ref Appendix B MIL STD 785B Task 205 requires the contractor to present results of SCA DI R 7083 is the applicable DID for delivering data under Task 205 By modifying DI R 7083 the PA will obtain SCA data which support existing system acquisitions and will contribute to a database which will support future system acquisitions Rationale for each change is as follows Block_3 Description Purpose The overall rewrite of Block 3 is recommended for clarity Changes point out that details of system design deficiencies assist the PA in evalu
79. nalysis Development and Application 1976 Region V IEEE Conference Digest 1976 pp 112 116 CLAR80 R C Clardy Sneak Circuit Analysis in J E Arsenault and J A Roberts ed Reliability and Maintainability of Electronic Systems Computer Science Press 1980 pp 223 241 MS1543B MIL STD 1543B Reliability Program Requirements for Space and Missile Systems Draft 29 September 1987 NP3634 Dept of Navy Sneak Circuit Analysis A Means of Verifying Design Integrity NAVSO P3634 stock nc 0518 LP 394 8000 July 1986 RANK70 J P Rankin and C F White Sneak Circuit Analysis Handbook Boeing report no D2 118341 1 NTIS no N71 12487 15 July 1970 88 5 EE ot ne A A A Annotated bibliography V D Baldwin Long Term Storage Effects on Fuzes AFATL TR 87 03 AD B110924L 26 January 1987 A review of design analyses including Stress Worst Case Safety Sneak Circuit that contribute to fuze design Benefits limitations and potential problems are evaluated for exch analysis Suggests ways in which SCA compliments Safety Fault Tree and FMEA Note Distribution of this document is authorized only by DoD agencies Boeing Co Electrical Analysis of B 52 FB 111 AMAC and Release Circuitry Utilizing Sneak Circuit Analysis Techniques report D2 118576 1 2 3 AD A103951 2 3 October 1975 Final report of a combined sneak circuit analysis and loading cable power shorting analysis Includes network
80. nce for these evaluations to the less experienced user The guidance is in form of prompts regarding the location of critical loads and the timing of switches affecting the path An example of a reverse current path identified by SCAT is shown by the schematic in Figure 3 2 The circuit is a simplified version of the infamous Redstone missile blockhouse interface that caused premature engine cutoff a few seconds after launch The cause was determined to be the sneak path highlighted on the schematic between the launch command and engine cutoff relays that occurred when the ground umbilical separated a fraction of a second before the separate power umbilical The net list for this schematic was processed by SCAT and the resulting screen corresponding to the reverse current path is shown in Figure 3 3 The path is identified by the part reference designators appearing in the schematic 3 2 3 Functionally Oriented Design Concern Analysis Functionally oriented design concerns address the following types of sneak conditions power to power ties inadvertent load power cutoff by logically AND d switching devices inadvertent load power enabling by logically OR d switching devices improper timing for power enabling and power cutoff misleading indications and labels More than serving as clues for the analyst these concerns compose a knowledge base of rules that are evaluated by SCAT with respect to the specific circuit being analyzed
81. nd unfortunately is not compatible with older versions An interface with the new version is therefore required 44 Appendix A SCAT USER S MANUAL Note This User s Manual has been written as a stand alone document and therefore contains information that may appear elsewhere in this Final Report where such information was also considered applicable for this manual A 1 INTRODUCTION Sneak Circuit Analysis SCA is an established procedure for identifying sneak related problems sneak paths sneak timing sneak labels indications design concerns drawing errors in electrical circuits The procedure is specified as Task 205 in MIL STD 785B where sneak circuits are defined as unintended paths that can cause an undesired function to occur or a desired function not to occur assuming no component failures A non topological version of SCA is specified in MIL STD 1543B where functional paths and design concerns are addressed Standard SCA procedures are highly labor intensive and process input data available only during the latter portion of the development cycle Systems have been developed for automating the data formatting portions of the procedure but these require expensive computer resources typically large batch processing systems and experienced analysts The Sneak Circuit Analysis Tool SCAT overcomes these deficiencies by providing a personal computer based system for real time identification of sneak paths and design concerns e
82. nded to identify many but not necessarily all sneak conditions early in the design when interfaces may not yet be completely defined In this manner problems can be corrected early at minimal cost so that at a later development phase a more conventional SCA can be performed on the entire system and uncover the few if any remaining problems 3 2 Description The automated procedure comprises four major tasks Schematic capture net list generation Sneak path analysis Functionally oriented design concern analysis Device oriented design concern analysis US These tasks have been computerized utilizing a concurrent engineering environment comprising a commercial schematic capture product and the expert system based SCAT Schematic capture and net list generation are performed by OrCAD SDT III version 3 21 or later release It is available through OrCAD Systems Corp Hillsboro Oregon The sneak path and design concern analyses are performed by the SCAT expert system developed from an M 1 Teknowledge Inc expert system shell The shell consists of the M 1 inference engine and facilities for developing and maintaining the SCAT knowledge base M 1 was selected from among eleven commercially available MS DOS based shells evaluated for the SCAT application M 1 was selected because of its execution speed the shell is coded in C rather than LISP which is used by many others its rich repertory of syntactical functions e g pattern matc
83. ng ria E OR ONCE OP qud y 47 Fig te A 2 SCAT Program EE AAN A 51 Figure A 3 Schematic Symbols for SCAT Net Lists o 54 Figure A 4 Additional Schematic Symbols o ooooooooo 55 Figure A 5 Types of Switching Devices o ooooooooo oo ooo oo 57 Figure A 6 Schematic of the Example Circuit o ooooooooooo o 65 Figure A 7 Net List Entry Screen s ea eb ER e A CC 66 Figure A 8 Main Menu a co rm vh A NEE Ehe 66 Fisute A 0 IC Message Eu A379 ded AS SUPR S C E DAA e 67 Figure A 10 Sneak Input Data Men 67 Figure A 11 Reverse Current Path 1 54 hk RACE RIA OS EE 68 Figure A 12 Path 1 Highlighted on Schematic o eee ee eee 68 Figure A 13 Reverse Current Path 2 agi AAA Se ACA RR 69 Figure A 14 Path 2 Highlighted on Schematic ccc eee ee ee ee ee 69 Figure A 15 Power Source Menu AN ANUS aia 91 4 we whale de E 70 Figure A 16 Ground List Menu PARA AREA AA A 70 Figure A 17 Circuit Type Menu IN 71 Figure A 18 Design Parameter Summary 71 Figure A 19 Design Concern Type Menu NN e hr e REA 72 Figure A 20 Switch Relay Model Menu ooooooooomoo omo 72 Figure A 21 Switch Configuration Menu 73 Figure A 22 Main Menu Listing M B B Switches o 73 Figure 4 25 Reverse Current Path 4 cia Pad dae Ee HA UPS 74 Figure A 24 Path Marked for Deletion oooooooo ooo ooo oo s 74 Figure A 25 Regenerated R
84. nmanned fixed or rotary wing a satellite a launch vehicle ground systems a new avionics or communications suite a computer or even a circuit board This field should contain the highest level title of the acquisition program or project Field 2 Designator This field contains the program designator for the vehicic o application in field 1 Using the examples above this could be B 2 BQM 126A or AH 64A aircraft DSCS 3 spacecraft Titan 4 launch vehicle OTH B ground systems AN ALQ 172 avionics Computer and circuit board could be listed by part number f Analysis or Test This field contains the type of analysis or test performed which identified the problem Examples are Sneak Circuit Analysis Fault Tree Analysis Finite State Machine Analysis Peer Code Review Burn In Test etc Field 4 Date Problem Identified This is the date recorded on the problem report and submitted through DI R 7080 This data field is significant since it is easily plotted against program milestone dates in File 1 This allows analysts to pinpoint precisely when during program acquisition each problem was identified Field 5 Title of Problem Report This should be the same title entered on the problem report and submitted through DI R 7080 40 Field 6 Vehicie or Application System This is the higher level system within the vehicle or application identified in field 1 Inertial Navigation System for example is a high level sy
85. o a subset of sneak clues e those associated with commonly encountered sneak conditions However the procedure can readily be extended to apply to a wider variety of input data and check for a larger number of conditions 3 1 Overview SCAT is a microcomputer based expert system for automatically identifying sneak paths and design concerns by processing circuit net lists generated by a CAD schematic capture tool SCAT differs from conventional SCA techniques in that the latter are based upon the generation usually automated and analysis mostly manual of network trees to identify sneak paths The proposed tool does not require network trees in fact it is particularly applicable to early phases of a design when detailed circuit data required for generating trees are not available The automated procedure provides the design engineer or reliability analyst with a simple tool for rapidly identifying and correcting sneak circuits and relevant design concerns Identification of topological patterns is not required Sneak paths are automatically identified for power switching circuitry Design concerns relevant to sneak circuits are identified for analog or digital circuits The procedure focuses the analysis on portions of the circuitry for which the analyst has design responsibility or detailed understanding of its operation e g a circuit card assembly or a subsystem such as power distribution A more extensive analysis would require applicati
86. odes have been specified and if there are no IC s or if there are and the user chooses to continue the sneak path input data menu appears The user may choose to 4 1 4 2 4 3 4 4 4 5 Delete parts to simulate OPENed switches remove redundant paths thereby reducing processing time delete parhs for some other arbitrary reason A list of the deleted parts and the reason for their deletion is maintained by the program Undelete previously deleted parts Change the source node for the analysis If a nade is labeled SRC on the schematic it is the default source Change the sink node for the analysis If a node is labeled SNK on the schematic it is the default sink Execute the sneak path search Sneak path processing will commence Processing time for small circuits i e short net lists is a few seconds larger circuits require more time The reverse current path menu will appear when processing is concluded Each path is identified by a list of device names corresponding to those in the circuit schemaiic that lie on the sneak path between the source and sink nodes Devices that lie on the bidirectional portion of the path are prefixed by an As an aid for cross referencing the path list to the schematic relays are listed with either a S suffix to indicate a switching contact or a C to indicate a coil The analyst can trace the path on a copy of the schematic to facilitate its evaluation The following options
87. on SIS F10 Command Menu Figure A 7 Net List Entry Screen M1 Commands PPLICATION DISPLAY demo net has been loaded Select SNEAK PATH SEARCH or DESIGN CONCERN ANALYSIS Prior to selection modify suitch relay and capacitor models if necessary All switches and relays will be modeled as BREAK BEFORE MAKE unless otherwise specified All capacitors will be nodeled as OPEN circuits unless otheruise specified UESTION Choose desired option Sneak Path Search Design Concern Analysis nodel suitch Model suitch nodel capacitor Hodel capacitor exit program Exit to DOS unknoun Space to Mark Figure A 8 Main Menu 66 M1 Commands PPLICATION DISPLAY The netlist contains ICs If sneak path analysis is performed the ICs will be ignored creating breaks in the circuit at these points If you do not wish to model the ICs in this manner return to the main menu at this time Alternate modeling can then be performed by exiting to DOS modifying the schematic to replace the ICs with switches and diodes regenerating the netlist and then running SNEAK PATH ANALYSIS NSUER Choose one of the follouing options Continue Sneak Path Analysis return Return to Main Menu unknoun Space to Mark ALT A Abort F2 Scroll Display F10 C mmand Menu Figure A 9 1C Message Mi Commands A PPLICATION DISPLAVy Source src
88. on of a conventional SCA However even in this case prior use of the proposed procedure would minimize the number of remaining sneaks and thereby greatly reduce the cost impact and other concerns associated with correcting problems late in the design phase The automated procedure is based in part on the fact that sneak paths involve circuit branches that can conduct current in either direction depending upon the switching state of the circuit SCAT searches for these bidirectional branches rather than perform the more complex task of searching for specific topological circuit patterns as done by conventional automated SCA techniques The analyst s task is also reduced to evaluating the significance of specific sneak paths rather than applying clue lists to circuit patterns for identifying the sneak paths 20 A significant issue that arises in regard to focusing the analysis at any one time to a portion of the system is the assurance that sneak paths associated with assembly or subsystem interfaces are not overlooked This issue is addressed in two ways First the system compels the user to identify each interface port of a switching circuit in terms of it being a power input ground return or signal I O Interfaces to power and ground are labeled as such regardless of whether they respectively go to power and ground directly or through switched or unswitched loads and they are included within the sneak path search Second the SCAT is inte
89. oughness of sneak circuit procedures the number of pertinent sneak problems found by other techniques are compared with the number of problems of equal severity that are found by SCA 4 Cost effectiveness studies The resources required to find a problem of a given severity level by SCA is compared to the resources required to find an equally severe problem by other techniques The investigative approach for these areas was based upon analyzing existing government and contractor requirements for identification and documentation of sneaks identified as part of a sneak circuit analysis The investigation encompassed an evaluation of 1 military standards 2 associated Data Items Descriptions DIDs and 3 previous studies directed at sneak circuit analysis SCA contractors have their individual internal policies and procedures for performing SCA and tracking resultant identified sneaks This complicated the study since contractor procedures could not be obtained The experiences of the investigators were utilized to overcome this problem 4 1 Background Overall reliability programs for government procurements and for which sneak circuit analysis may be a part are generally conducted in accordance with MIL STD 785B Reliability program reviews are required by Task 103 of the standard This task permits the contractor and the Program Authority PA to review overall program status including The Program Authority is the responsible government
90. path has been opened In the latter there is some overlap for a short period typically a few milliseconds during which time the newly selected path and the old path exist concurrently Models corresponding to these two configurations can be specified for switches and relays either individually or globally Capacitor terminals can be modeled as being unconnected to each other i e OPEN or connected together i e SHORTed The former the default case applies to paths involving DC currents while the latter applies to AC or transient current paths As before models corresponding to these two configurations can be specified for capacitors either individually or globally 4 Option 1 Select sneak paths If the net list includes IC s the user is informed that sneak path processing will treat paths to these devices as open circuits The user has the option of continuing with the analysis or returning to the main menu If both a source node and sink node have not been specified in the schematic the user is requested for these data 56 Ee a A Single Pole Single Throw SPST switch is inherently Break Before Make o Aen b This Single Pole Double Throw SPDT switch is Breok Before Make 2 c This SPDT switch is Make Before Breok Current will momentorily flow out both contacts 1 and 2 as the switch is actuated Figure A 5 Types of Switching Devices 57 When source and sink n
91. ponsible for circuit design the functional and device guidelines are intended for either an engineer or an analyst familiar with the operation of the circuitry and its constituent devices The functional guidelines may be applied before specific circuit device types have been finalized Their application requires a diagram depicting power distribution at the assembly e g printed circuit board level and identification of analog or digital and high or low current areas of the circuitry The device guidelines are applied after circuit devices have been selected for the Jesign The selection need only be generic e g NPN bipolar transistor low power Schottky TTL digital counter specific part numbers are not necessary For complex systems application of the guidelines should be focused on circuitry associated with critical system functions rather than the entire system Critical functions may be identified by fault tree analysis or FMEA The manual procedure is documented in a report titled Sneak Circuit Analysis for the Common Man report number RADC TR 89 223 generated as a part of this study An example of a design rule appears in Figure 2 2 and an example of a guideline appears in Figure 2 3 Rule 1 MULTIPLE POWER SOURCES AND RETURNS PROBLEM Sneak paths involving multiple power sources and or multiple ground returns SOLUTION Structure circuits so that all current for a given load flows from one power source to one ground re
92. potential reliability problems introduced by Government furnished equipment and descriptions of accommodations or improve ment changes deemed necessary to make such equipment compatible 2 The report shall include a graphic discussion of trends A breakdown to the configuration item level shall be made in the following manner Allocated Predicted Observed Recuirement Value Value Valve 3 The report shall include proposed changes to the Reliability Program Plan as applicable 4 The Final Status Report can be identified as the Procra Summary Repert Page 2 of 2 Pages Exhibit C 1 Continued Page 2 of 2 85 DATA ITEM DESCRIPTION 3 DESCRIPTION PURPOSE DI R 7080 Rev A 4 APPROVAL DATE To monitor and evaluate contractor s progress and accomplishments in conducting the Reliability Program for the applicable contract end item s Y APPLICATION INSERRELATIONSHIP S OFFICE Of PRIMARY RESPONSIBILITY USAF DOC REQJIRED 8 APPROVAL LIMITATION Applicable to contracts which contain the requirements for reliability Program Reviews in accordance with MIL STD 7853 Task 103 9 REFERENCES Mandatory as Cited in block 10 MIL STD 785B Task 103 This DID supersedes DI R 1731 and Di R 2119 10 PREPARATION INSTRUCTIONS 1 Each report shall include the following information as a minimum 8 The work accomplished and results obtained on each task defined by the work statement or the Contr
93. r Break Before Make will also be demonstrated Figure A 29 depicting SCAT program flow references the screens described in the following example 1 Install SCAT on to hard disk as described in the SCAT Installation Procedure 2 At the DOS prompt enter lt SCAT gt The net list entry screen Figure A 7 will appear 3 At the net list entry screen enter lt demo net gt The main menu Figure A 8 will appear 4 At the main menu select sneak The IC message Figure A 9 will appear 5 At the IC message select continue The sneak input data menu Figure A 10 will appear 60 6 At the sneak input data menu select execute The first reverse current path will appear 7 At the reverse current paths menu Figure A 11 observe Path 1 data The path number appears at the top line of the APPLICATION DISPLAY window Observe that a total of two reverse current paths were identified A system reference number for the path also appears Since no parts were deleted the DELETED PARTS data appearing below the path listing are all empty i e Path 1 is K1 S S2 K1 C The path shown highlighted in Figure A 12 comprises the source labeled SRC on the schematic a switch contact on relay K1 the switch S2 the coil of relay K1 and ground sink labeled SNK on the schematic The source and sink nodes do not explicitly appear in the path list but are implied Select next and previous and observe the path numb
94. r distribution and ground return circuitry SOLUTION Use the same circuit connection topology for the supply side and ground side of a load Use the same connector for symmetrical power and ground connections COMMENT Circuit connection symmetry for power and ground distribution implies an identical number and location of power and ground connections feeding a load Asymmetrical connections can cause sneak paths as shown in Figure 10 In part A of the figure power connection J3 has no counterpart on the ground side of load X2 If connections J2 and J3 are open while the remainder are closed current can unintentionally flow in the reverse direction through X2 This problem has been eliminated in part B of the figure by the inclusion of connection J3 2 A PROBLEN B SOLUTION Figure 10 SYMMETRICAL POWER DISTRIBUTION Figure 2 3 Guideline Example 19 sd Chapter 3 AUTOMATED SCA This chapter describes the development and operation of a simplified automated SCA procedure developed during this study The intent of developing this procedure was to demonstrate the concept and feasibility of integrating an SCA tool with an automated design tool to provide a simple yet effective sneak analysis procedure To this end the procedure and the automated SCAT supporting it were constrained to a specific input domain i e a net list comprising circuit device types from a specified parts library and formatted in a specified manner and t
95. r the following list which will be modeled MAKE BEFORE BREAK s3 k1 All capacitors will be nodeled as OPEN circuits unless otheruise specified UESTION Choose desired option Sneak Path Search Design Concern Analysis nodel suitch Model suitch nodel capacitor Model capacitor exit program Exit to DOS unknoun Space to Mark ALT A Abort Fe Scroll Display F10 Command Menu d Figure A 22 Main Menu Listing M B B Switch 73 Mi Commands PPLICATION DISPLAY demo net src to snk reverse current path 4 of 4 path ref number 3 Q1 SZ2 K1 S LP1 NOTE Asterisk Gei indicates a reverse current device Deleted Parallel Parts 1 Deleted Switch Related Parts 1 Deleted Miscellaneous Parts UI Use up down arrows to scroll answers NSWER Display Next Path next Display Previous Path previous Analyze Current Path Delete Current Path delete Print Path print Return to Menu return Space to Mark ALT A Abort F2 Scroll Display F10 Command Menu Figure A 23 Reverse Current Path 4 Mi Commands demo net src to snk reverse current path 4 of 4 path ref number 9 1Q1 S2 K1 S LP1 NOTE Asterisk G9 indicates a reverse current device Deleted Parallel Parts l Deleted Suitch Related Parts 1 Deleted Miscellaneous Parts 1 lt lt THIS PATH IS MARKED FOR DELETION gt gt No sneaks uere identified for this path NSWER Regenerate Paths regenerate paths Undelete Current Pa
96. recommendations for data collection to assist in managing and evaluating contractual SCA efforts Chapter 5 Recommendations for Further Study addresses areas that were beyond the scope and resources of the current study A user s manual for the automated SCA tool SCAT and proposed data items for control and monitoring of SCA appear in appendices to this volume of the report The SCAT software consisting of an expert system knowledge base augmented by programs coded in C is documented in Volume II of this report The documentation consists of printouts of the C source code and knowledge base both of which include extensive comments and descriptions of the program flow and data structures utilized For ease of reference each major subprogram and knowledge base segment is listed in the table of contents for Volume II This study was a two year effort commencing on September 1987 SoHaR Incorporated the prime contractor for this study was responsible for developing the manual and automated procedures described in the report and for writing the C code and MI knowledge bases required for SCAT Fail Safe Technology Corporation was the principal contributor for development of the control and monitoring procedures An analysis and evaluation of candidate expert systems for the automated procedure was performed by Dr Lawrence Press a consultant to SoHaR Overall technical direction was provided Mr Bruce Dudley RADC RBER Chapter 2 BACKGR
97. rgeted for use early in the design phase as well as for existing systems 11 R amp M STD RO00205 Sneak Circuit Analysis Naval Avionics Center Reliability amp Maintainability Std 29 May 1986 Government standard for performing hardware and software sneak analysis Includes recommended phasing and examples of SCA projects completed reports contractual SoWs DIDs 90 Dept of Navy Contracting and Management Guide for Sneak Circuit Analysis SCA NAVSEA TE001 AA GYD 010 SCA AD A094541 September 1980 Addresses SCA benefits problems cost factors tradeoffs comparison with other analyses integration with CAD CAE manual vs automated techniques Includes examples of SOWs and DIDs Dept of Navy Sneak Circuit Analysis A Means of Verifying Design Integrity NAVSO P3634 stock no 0518 LP 394 8000 July 1986 SCA implementation details for a generalized computer aided procedure Includes cost estimation clue lists analysis example and other applications for the network tree data base Principle contributors were General Dynamics amp Boeing J P Rankin and C F White Sneak Circuit Analysis Handbook Boeing report no D2 118341 1 NTIS no N71 12487 15 July 1970 Methodology and specific details of SCA procedure used on Apollo program Describes data encoding circuit partitioning network tree generation and a basic clue set P F Stokes and G Buckley Sneak Circuit Analysis of FMU 112 B Fuze
98. round are labeled as such regardless of whether they respectively go to power and ground directly or through switched or unswitched loads and they are included within the sneak path search It is important to note that SCAT is intended to identify sneak conditions early in the design when interfaces may not yet be completely defined In this manner problems can be corrected early at minimal cost so that at a later development phase a more conventional SCA can be performed on the entire system and uncover the few if any remaining problems A 2 THE AUTOMATED PROCEDURE The automated procedure for performing SCA consists of the following steps 1 Target critical areas of a system for analysis The requirement for performing a SCA should apply to portions of a system considered critical These subsystems can be identified from the results of other analyses such as FMEA or Fault Tree Bear in mind that sneak path analysis addresses combinatorial power switching and distribution circuits and that design concern analysis although applicable to all analog and digital circuitry is not intended for identifying sneak paths To insure thoroughness all external interfaces of the targeted subsystems must be defined in terms of being dedicated power or ground switched power or ground or signal lines 2 Partition the circuitry to be analyzed into manageable segments The appropriate size of a segment is a function of the following constraints a The a
99. rs The rule checking code however is in a closed format that cannot be modified without detailed knowledge of the entire schematic capture program Alternatively the net list output of the captured schematic can be used as an input to an external program for automating the SCA checklists This latter approach is the one chosen for SCAT and is described later in this report Of the products surveyed OrCAD SDT was the one selected for integration with the automated SCA tool because of its compatibility with MS DOS low cost wide popularity and provisions for generating net lists in an industry standard format specifically Electronic Data Interchange Format or EDIP Earlier work by SoHaR demonstrated an advantage for using expert system technology to aid the application of SCA clues This technology is central to the automated version of SCA being developed under this study A survey of PC based expert system shells was undertaken to facilitate the selection of an optimum tool for automating the analysis In addition eleven expert system shells were evaluated in terms of execution efficiency development efficiency user interface developer interface external interface inference process knowledge representation and developing company policy regarding the use of the shell as part of another product 2 2 2 User Survey A survey of vendors of SCA and of those who require or specify its performance was undertaken to determine the current st
100. rs were contacted and seven provided useful information by responding to a survey questionnaire The findings indicated network trees are required for a comprehensive search for sneak paths However these trees are difficult to implement in that they require significant data processing performed by proprietary software for 1 generating a circuit connectivity list net list defining the entire system 2 filtering redundant or non essential information from the list and 3 partitioning the net list into segments suitable for manual application of the clues This study concentrated on developing guidelines for identifying sneak circuits that are independent of circuit topology thereby eliminating the need to generate network trees Additional findings of the user survey were The current prevalent procedure for SCA consists of automated formatting and partitioning of schematic and net list data semi automatic generation of network trees and manual application of sneak clues and design concerns Efforts are underway for reducing the requisite computer resources from a mainframe to a workstation lt Functional networks e g block diagrams are rarely analyzed vi The most prevalent types of analys s for Which SCA databases and results are shared are FMEA and fault tree analysis The last point was supported by a survey of related analyses conducted as part of this study The analyses survey also found that fault tol
101. s that have developed an SCA capability Furthermore a team of specially trained analysts are required to apply sneak clue lists many of the lists are considered proprietary as well to the hundreds of network trees that are typically generated For these reasons performance of the analysis is limited to SCA contractors in all but the simplest of cases 2 2 Interim Results In order to build a foundation for the development of a simplified integrated version of SCA a data collection task was performed The task consisted of three major activities 1 A literature search 2 A user survey 3 A survey of related analysis techniques The collected data was evaluated in the specific areas of clue lists SCA techniques and related analyses The data and the evaluation results are summarized in the following sections 2 2 1 Literature Search and Analysis The literature search identified existing available information related to sneak analysis techniques methods of execution and problem areas Sources included the DTIC NTIC and UCLA library data bases The search also identified design tools that could be integrated with SCA to enable a design engineer to perform the analysis as an ongoing part of the design process The tools investigated were 1 computer aided design products presently being used for electronic equipment design and 2 expert system building tools for test and analysis applications A listing of significant references appears
102. s to problems being incorrectly identified or to identified problems not being corrected The objective of this study was to overcome these deficiencies by simplifying the procedure and integrating it with other analyses performed during the design phase To this end we 1 conducted a literature search and a SCA user survey to ascertain current methodologies and techniques associated with SCA and its support of other reliability analyses 2 developed a simplified manual procedure which provides design rules for avoiding sneak paths and guidelines for identifying common types of sneak conditions and 3 developed an automated version of the procedure integrated with a popular computer aided design CAD tool The procedures are intended for the reliability analyst or design engineer without prior SCA experience The results of these three tasks are described in the remainder of this report Chapter Background and Interim Results summarizes conventional SCA procedures and briefly discusses objectives and accomplishments during the first half of this contract including development of a new manual procedure Chapter 3 Automated SCA describes the development and operation of the new automated procedure its integration with existing design tools and its relevance to other reliability analyses Chapter 4 SCA Control and This report does not address a related procedure for software Sneak Software Analysis Monitoring presents
103. s used by various KBases Power power paths identified by PATHS for FUNCTION Flag generated by SCAIN for SCAT to execute SCA 50 ASCAT BAT D SCAFILE K SCAFILEB ter SCAIN DESIGN SCA PATHS C C SCAOUT i DSGNMENU K K FUNCTION DEVICE K K Legend C C code D DOS K Knowledge Base Figure A 2 SCAT Program 51 The main menu allows the user to modify switch relay and capacitor models before choosing one of the following types of analyses 1 Sneak Path Search performed by the C program SCA and the knowledge bases SCAIN and SCAOUT 2 Design Concern Analysis performed by the C program PATHS and the knowledge bases DESIGN DSGNMENU FUNCTION and DEVICE Sneak path search applies only to switching circuitry i e circuitry consisting of current interrupting devices switches relays connectors circuit breakers fuses Sneak paths are not traced through integrated circuits IC s are instead automatically modeled by SCAT as open terminations Ge IC leads are treated as open circuits However by using OrCAD to edit the schematic the user can substitute equivalent circuits if he is aware of them SCAT searches for potential sneak paths by first identifying all directed non circular topological paths between two user specified nodes the source node starting point and the sink node ending point These paths are then analyzed to identify those that are bi
104. selection Called by SCAT Calls DESIGN or SCAIN Displays sneak paths Assists user evaluation Calls PATHS Called by SCAT 49 File Name BIS SCA DATA MOD DATA PTH DATA SCA DATABAK MOD DATABAK SCA DELS SCA DEVS SCA DONE DSN DONE SCA FILE SCA JOINS SCA PATHS PTH SNEAK SCA File Type Size ASCII ASCII ASCII ASCII ASCII 03K ASCII 09K ASCII ASCII ASCli ASCII ASCII ASCII ASCTi ASCII Varies with circuit size Program flag Memory requirement is negligible Table A 2 SCAT Data Files Description Sneak paths identified by SCA for SCAOUT Generated by SCAMENU Contains switch relay and capacitor models Power sources identified by DESIGN for use by PATHS Also used as a flag file for SCAT to execute PATHS Generated by SCAIN Lists deleted net list parts and Source sink nodes Initial system data for switch relay and capacitor models Initial system data for source sink nodes Generated by SCAOUT and used by SCA Contains deleted paths Generated by EDIF2MI Identifies all net list parts and ports The file is used by various KBases Flag generated by DSGNMENU for SCAT to execute SCAMENU Flag generated by SCAMENU for SCAT to terminate and exit to DOS Contains net list file name generated by SCAFILE or SCAFILEB for EDIF2MI and SCA Generated by EDIF2M1 Identifies circuit nodes in the net list The file i
105. space 35 Company In part it stated that A major element missing from this Boeing effort which could be considered in measuring effectiveness is a method to track the resulting dispositions for the Sneak Analysis Reports The report went on to say that only by collecting relevant data could SCA effectiveness be measured In order for the PA to assess SCA effectiveness all sneaks must be tracked from identification through resolution and disposition Collection of tracking data depends on requirements in the contract Statement of Work SOW since there is no DID associated with a monitor and track task DI R 7083 contains general information regarding the SCA Report content with no specific instructions with regard to items of interest to the PA Data collection regarding each identified sneak should logically be performed by the contractor These data include elements such as sneak type category nature of the sneak sneak severity category disposition and manhours and costs to implement design changes A weighting factor based on the sneak severity category should be applied to weigh the relative significance of each identified sneak 4 2 3 SCA Cost Effectiveness Relevant data for analyzing cost effectiveness are not generally available In addition to SCA data submitted by the contractor two cost data elements must be added by the PA before a meaningful analysis is possible These elements are 1 total contract cost to per
106. spect to FMEA probably the most widely invoked task of those shown in Table 2 4 there is much latitude in the level of detail that is required to be covered and therefore uncertainty about the usefulness of close integration with SCA A summary description of interfaces between SCA and related analyses is presented in Table 2 5 The greatest potential for duplication and therefore also for cost reduction exists between SCA and the operational group of analyses Worst case and sensitivity analyses cover many of the areas that are included in the design concern analysis part of SCA Worst case analysis considers system performance when component tolerances and environmental conditions are at their specified extreme limits Sensitivity analysis evaluates the degree to which system performance is affected by small variations in the values of the system components Where compliance with fault tolerance design criteria must be analyzed MIL STD 785B par 50 2 4 1 this also has an important interface because the criticality of the functions that makes SCA necessary will in most cases also require fault tolerance and the latter must address the absence of design factors such as sneak circuits that can defeat its purpose Single point failure analysis an important part of fault tolerance analysis has significant interfaces with sneak circuit analysis Grounding analysis which is really a design rather than reliability technique covers one of t
107. splay Next Path next Display Previous Path Analyze Current Path analyze Delete Current Path delete Print Path print Return to Menu return Space to Mark ALT Abort F2 Scroll Display F10 Command Menu Figure A 13 Reverse Current Path 2 Figure A 14 Path 2 Highlighted on Schematic 69 Mi Commands PPLICATION DISPLAY Identified external power sources ac_pur ac_rtn src x_plusSudc Verify the list It must include all AC and DC EXTERNAL voltage sources DO NOT APPEND any internal Cin circuit voltage sources e g in circuit batteries To append a nane to the list 1 Type louer case 2 Replace spaces with underscores Li Spell out and i e plus and ninus Any nane starting with a number a vi or a with x eg 5u dc gt x_plusSu_de A 3 should be prefixed NSUER Choose one of the follouing no change delete delete a name from present list append append a nane to present list unknoun Space to Mark ALT A Abort F2 Scroli Display F10 Command Menu Figure A 15 Power Source Menu Mi Commands Identified grounds Esnk Make sure the list is correct NSUER Choose one of the follouing no change delete delete a name from present list append append a name to present list unknown Space to Mark ALT A Abort FZ Scroll Display F10 Command Menu Figure A 16 Ground List Menu 70 M1 Commands PPLICATION DISPLAY QUESTION Does th
108. stem within the vehicle F 111 Field 7 Vehicle or Application Subsystem This is the subsystem within the system identified in field 6 In the above example a stable platform may be a subsystem within the Inertial Navigation System Field 8 Hardware or Software This field identifies the problem as hardware software or both Fiel 9 Hazard Category This is the hazard category as determined by the analyst at the time the problem was identified It will be either Category I Catastrophic Category D Critical Category UI Marginal or Category IV Minor Dei 10 Disposition This field is used to note the problem disposition as design change or no change Field_11 Manhcurs to correct Problem Includes all program hours direcied toward correcting the problem Field 12 Costs to Correct Problem Includes all program costs incurred during the process of correcting the problem 44 3 Summary of SCA Data Collection and Analysis Requirements The database that results from his data collection effort will support further analyses and will support drawing important conclusions This is possible where reports have been generated for providing data described in Section 4 3 and surnmarized by the checklist appearing in Table 4 1 Comparative analyses can then be performed to determine which type of analysis or test results in the greatest number of identified problems Inclusion of the program phase and problem resolution costs gives even
109. ters will appear 39 Select continue The design concern type menu will appear 40 At the design concern type menu select functional guideln The function oriented design concern analysis will commence 63 41 The first function oriented design concern will now be a warning regarding a power to power tie Figure A 27 The corresponding sneak path appears in the message and may be traced on the circuit schematic see Figure A 28 Note that this warning did not appear earlier when switch S3 and relay K1 were modeled as Break Before Make As before explanation solution or warning message may be selected 42 Return to the design concern type menu by either selecting return where available as a choice pressing ALT A or repeatedly answering queries and selecting continue until the analysis concludes and then selecting return 43 At the design concern type menu select return The main menu will appear 44 At the main menu select exit_program The DOS prompt will appear WIAD aide ou JO ILWIS 9 Y am P51 s04202U Mengs CUO AR Put 4 estie t To VESENT va BIEENE to wr CID a 65 Mi Commands PPLICATION DISPLAY SCAT Ver 1 3 Developed By SoHaR Inc Developed For RADC RBER c n F30602 87 c 0193 COTR B Dudley 315 330 2608 Release Date 10 2 83 NSUER Enter the name of the net list file Enter lowercase expressi
110. th undelete unknoun Space to Mark ALT A Abort F2 Scroll Dfsplay F10 Command Menu Figure A 24 Path Marked for Deletion 74 ML Commands Ldemo netl src to snk reverse current path 1 of 2 path ref number 6 K1 S x S2 K1 C NOTE Asterisk indicates a reverse current device Deleted Parallel Parts 1 Deleted Switch Related Parts Deleted Miscellaneous Parts Use up down arrows to scroll answers Display Next Path Display Previous Path previous Analyze Current Path analyze Delete Current Path delete Display Paths Deleted Earlier eleted_paths l Print Path ALT A Abort EG Scroll SIEUT M1 Commands PPLICATION DISPLAY demo net src to snk lt lt DELETED gt gt path 1 of 1 path ref number 9 1Q1 S2 K1 S LP11 NOTE Asterisk is NOT INCLUDED Deleted Parallel Parts Deleted Suitch Related Parts Deleted Miscellaneous Parts II NSUER Display Next Path next Display Previous Path previous Unde lete Delete undelete Display Regenerated Paths Print Path print unknoun Space to Mark ALT A Abort FZ Scroll Display F10 Command Menu Figure A 26 Deleted Paths Screen 15 Mi Commands Check for possible power to power tie between src and ac_pur when suitching devices are closed The sneak path s comprises the following devices path 1 K1 S3 K1 S1 F1 NSUER Choose one of the following options explanation solution warning_message retur
111. the previous paragraph was tested by analyzing topologically circuitry having no reverse current paths Case Das well as circuitry with above average numbers of such paths Case II and III In all cases all seeded reverse current paths were found and no path was mis identified as being a reverse current path Table 3 1 Test Results Computer uP RAM Clock Perf Test Reverse Search Ratio Case Paths Time 286 640 KB 10 MHz 11 5 0 0 20 s iH 4 4 35 s ul 8 8 85 s 386 1 MB 16 MHz 18 0 0 15s II 4 4 25s I 8 8 50 s Performance compared to standard IBM PC Seeded paths identified paths Figure 3 9 Test Case 1 31 E M Ni iki N eH m ar Neu IK fs IN d BU li d i 0099088 siiki Test Case MI Chapter 4 CONTROL AND MONITORING OF SCA A major objective of this study was to develop control and monitoring procedures for evaluating the effectiveness of an SCA effort Four areas were investigated in this regard 1 Sneak report follow up procedures These serve as a procedural control for assuring that problems identified by sneak circuit analysis are considered but not necessarily acted on by those responsible for the design 2 SCA effectiveness accounting This reporting area is intended to capture the impact of sneak analyses on design by measuring the number and significance of changes that are implemented as a result of SCA gt Thoroughness monitoring To measure the quality and thor
112. to this report as is a detailed user s manual for the SCAT program vii ei Zem 5 oot ow KT D zo ve RIOUS Er LUS ainpadoig pojeurojny ay q payyuepy ye d xesug y andy V 4HT vo LANZ AIR ut A3 O SESNZ To Chapter 1 INTRODUCTION This rep rt documents the results of a two year study to make sneak circuit analysis SCA more effective by simplifying the procedure and integrating it into the design process Sneak circuit analysis is defined in MIL STD 785B the military standard for reliability programs as a procedure to identify latent paths which cause occurrence of unwanted functions or inhibit desired functions assuming all components are functioning properly The procedure is particularly applicable to military systems because of the potential for identifying and correcting design weaknesses that could lead to catastrophic failure However the procedure is not as widely used as it should be primarily because Its conduct is expensive being highly labor intensive and often requiring an independent contractor having specialized tools and trained analysts The effort requires complete documentation and therefore is usually performed late in the design cycle or early in the production phase when changes are more costly and difficult to implement The separation of the organization performing the analysis from the organization responsible for the design often lead
113. transistor and into load X1 when collector power Vec is removed by opening switch S1 The corresponding SCAT message identifying this concern appears in Figure 3 8 Note that the subject of the concern is identified in the warning message by its schematic designation and that the user has the option of requesting an explanation or solution in addition to the original warning message 3 3 Test Results The performance of the automated procedure was characterized by processing net lists seeded with sneak problems to 1 determine whether all sneak conditions were identified 2 determine if any non sneak conditions were identified and 3 measure the time required for performing the analysis The first two items were evaluated by applying SCAT to various circuits seeded with a known set of sneaks Time measurements were only for sneak path searches design concern analyses are heavily user interactive and any time measurements would inevitably be affected by the specific design concerns identified and the response time of the user In general design concern screens are updated 1 to 10 seconds after user input depending upon the number of concerns the system tests prior to displaying the next screen the complexity of the schematic being evaluated and the speed of the computer used to run the program User input typically requires under five seconds for data entry For the sneak path search elapsed time was measured from user entry of the path searc
114. turn DC power input DC power input AC power input corresponding AC return Option 2 Select design concerns analysis The following messages will appear 5 1 Power ground message The analysis requires that power and ground paths be unambiguously identified Automatic identification of the paths is initially attempted The user is then asked to validate the power and ground listings and correct them if necessary Additions or deletions are made by entering power or ground names one at a time As described in the message the names must appear as labels on the schematic and must be entered in lower case text Spaces within the name must be replaced by underscores Names prefaced by a number must be prefixed by x_ 5 2 Circuit message The user must designate the circuitry being analyzed as either analog digital or both Prior to continuing the currently identified power and ground nodes and the circuit type are displayed along with an option to modify them If the design concerns analysis is repeated this summary screen is displayed in place of the power ground and circuit type screens 59 5 3 Design concerns type message The analysis is divided into two parts functional guidelines and device guidelines to facilitate focusing on specific attributes of the circuit for analysis or re analysis The user is requested to specify the analysis type From this point on queries will arise as the system attempts to identify
115. turn Where this Is not possible isolate power sources using diodes for DC power or relays electromechanical or solid state for AC or DC power Use Schottky diodes or relays for DC applications requiring very low voltage drop and power dissipation Isolate returns by separating high and low current loads PAR A 2 D Ki 2 Si t t S s2 s3 g GND GND S s2 ALT GNO ALT GN ALT WR PRR P PWR 1 3 GNO ALT GND GND ZA RECOMNEN D ALTERNATIVE lt 4 RECOMNENOED 8 UNDESIRED C DESICN 0 ALTERNAT SOLUTION Figure 3 MULTIPLE POWER SOURCES AND RETURNS Adherence to this rule avoids Y X and Hr circuit patterns associated with multiple power sources and sinks see Chapter 2 This is a general rule to be followed wherever possible An example of a network complying with this rule appears in Figure 3A and an example of a network violating it appears in part B of the figure The violations shown can result in power to power or ground to ground ties Isolation must be provided to avoid the mixing of low current and high current ground returns Examples are shown in parts C and D of the figure Figure 2 2 Design Rule Example 18 POWER DISTRIBUTION CIRCUITS TARGET Primary and secondary power distribution circuitry comprising power sources ground returns switches contactors relays circuit breakers fuses solid state switches connectors PROBLEM Asymmetrical pattern of connections for powe
116. un Space to Mark F2 Scroll Display F10 Command Menu Figure 3 6 Explanation and Solution Messages 27 3 2 4 Device Oriented Design Concern Analysis Device oriented design concerns address sneak related probl ms associated with specific analog and digital devices appearing in the net list Concerns covered by the analysis include Reverse current paths through bipolar transistors Reverse current paths through op amp summing junctions Sneaks caused by noise sensitive devices Relay suppression problems Interface problems at the input and output of digital devices As was the case for the functional concerns the analysis was automated by translating each concern into a set of rules for the expert system knowledge base SCAT checks each device in the net list against these rules querying the user for additional information regarding the devices whenever conclusions cannot be drawn based solely upon data contained in the net list The user interface is identical to the functionally oriented concerns in that separate display windows are provided for queries user response menus and results Circuit components are referenced by their schematic designations and an explanation or solution can be requested for each identified concern An example of a design concern appears in Figure 3 7 As seen in the figure a sneak current can flow from signal generator through the base to collector junction of the NPN bipolar
117. was gathered on reliability analysis techniques that are prospective candidates for integration with SCA The more widely used of these analyses are FMEA fault tree analysis worst case analysis and preliminary hazard analysis It was observed that activities currently being conducted under the heading of sneak circuit analysis interface with and partially duplicate the above analyses and other reliability safety and design tasks The nature of the interface the data and techniques that may be common and the allocation of currently duplicated or undefined responsibilities were evaluated during this study 13 In this connection it is convenient to divide the reliability centered activities into those that relate to operational non failed equipment those that relate to failed equipment and those that relate to safety This division is shown in Table 2 4 A problem in the integration of sneak analysis with all of these is that on a given project there is no certainty that any of the other analyses are being conducted Table 2 4 Equipment Concerns of Reliability Analyses Worst Case Analysis Failure Modes amp Effects Analysis Preliminary Hazard Analysis Component Tolerance Analysis Fault Tree Analysis System Hazard Analysis Fault Tolerance Analysis Common Cause Failure Analysis Operations Hazard Analysis Sensitivity Analysis Fault Hazard Analysis Power amp Load Analysis Accident Analysis Grounding Analysis With re
118. y was to identify methods by which an automated SCA tool could easily interface with a schematic capture tool and to select a specific schematic capture product for implementing this interface These products accommodate on screen graphical and textual editing of circuit schematics and include provisions for outputting circuit interconnection data in various formats for use by other products such as those for circuit board trace routing and circuit or logic analysis Methods of integration that were considered included feedback of SCA results into the on screen schematic incorporation of SCA clues into the design error checking facilities found in many of the products and use of the circuit interconnection data the schematic net list as an input for the SCA tool A voluminous amount of information on CAD techniques is available in the literature In order to focus on the study objectives the scope of the search was limited to schematic capture products capable of running on a personal computer The personal computer e g the IBM PC family of desktop microcomputers is becoming prevalent in engineering organizations as its computing performance and memory capacity have increased and a growing number of circuit design and analysis applications are available that take advantage of these gains PC based schematic capture and circuit analysis products provide a cost effective means for interfacing SCA with design data particularly during the early phas
119. ytical tools tests applications and results will over time provide the foundation for improving system acquisitions Rationale for each proposed changes to the DID are as follows Block 10 Preparation Instructions The change to Block 10 only affects paragraph 1 c The change is intended to expand visibility into the overall Reliability program report presented at program reviews The last item on the original list 1 c 7 is relocated to 1 c 2 c in the revised DID 4 4 Automating SCA Evaluation A valuable tool for future program acquisitions is the knowledge and experience gained from past programs The control and monitoring study has dealt primarily with sneak circuit analysis and its impact on program acquisition Data gathered as part of SCA as well as other analyses and various testing methods could contribute to a database which would provide program managers with insight into costs and benefits of analyses and tests on past programs This insight then becomes a valuable tool which may be used to determine and apply the right analysis and test on future acquisitions In order to implement this process a database must be developed which contains relevant data Two types of data are required and each should be maintained within its own file The first file File 1 contains program data consisting of information about programs and their significant milestones The second file File 2 contains analysis and test data and proble

Integration of Sneak Analysis with Design

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